Correctly handle terms include/exclude

for Terms aggregation
- the flags property is no longer supported and a regular expression pattern is sent as a string value against the "include" key
- exact values are sent as an array of strings

for Significant Terms aggregation
- a dictionary was incorrectly used to filter terms. Introduce a SignificantTermsIncludeExclude type to represent the available options. Don't reuse the TermsIncludeExclude type because it includes properties that are not applicable for filtering on Significant Terms aggregation.

Add unit and integration tests for include and exclude filtering

Fixes #2682
Fixes #2676

Author: russcam

docs/aggregations/bucket/significant-terms/significant-terms-aggregation-usage.asciidoc

@@ -15,6 +15,18 @@ please modify the original csharp file found at the link and submit the PR with
 [[significant-terms-aggregation-usage]]
 == Significant Terms Aggregation Usage
 
+An aggregation that returns interesting or unusual occurrences of terms in a set.
+
+[WARNING]
+--
+The significant_terms aggregation can be very heavy when run on large indices. Work is in progress 
+to provide more lightweight sampling techniques. 
+As a result, the API for this feature may change in non-backwards compatible ways
+
+--
+
+See the Elasticsearch documentation on {ref_current}/search-aggregations-bucket-significantterms-aggregation.html[significant terms aggregation] for more detail.
+
 === Fluent DSL Example
 
 [source,csharp]
@@ -80,3 +92,153 @@ sigNames.Should().NotBeNull();
 sigNames.DocCount.Should().BeGreaterThan(0);
 ----
 
+[[significant-terms-pattern-filter]]
+[float]
+== Filtering with a regular expression pattern
+
+Using significant terms aggregation with filtering to include values using a regular expression pattern
+
+=== Fluent DSL Example
+
+[source,csharp]
+----
+s => s
+.Aggregations(a => a
+    .SignificantTerms("significant_names", st => st
+        .Field(p => p.Name)
+        .MinimumDocumentCount(10)
+        .MutualInformation(mi => mi
+            .BackgroundIsSuperSet()
+            .IncludeNegatives()
+        )
+        .Include("pi*")
+    )
+)
+----
+
+=== Object Initializer Syntax Example
+
+[source,csharp]
+----
+new SearchRequest<Project>
+{
+    Aggregations = new SignificantTermsAggregation("significant_names")
+    {
+        Field = Field<Project>(p => p.Name),
+        MinimumDocumentCount = 10,
+        MutualInformation = new MutualInformationHeuristic
+        {
+            BackgroundIsSuperSet = true,
+            IncludeNegatives = true
+        },
+        IncludeTerms = new SignificantTermsIncludeExclude("pi*")
+    }
+}
+----
+
+[source,javascript]
+.Example json output
+----
+{
+  "aggs": {
+    "significant_names": {
+      "significant_terms": {
+        "field": "name",
+        "min_doc_count": 10,
+        "mutual_information": {
+          "background_is_superset": true,
+          "include_negatives": true
+        },
+        "include": "pi*"
+      }
+    }
+  }
+}
+----
+
+=== Handling Responses
+
+[source,csharp]
+----
+response.ShouldBeValid();
+var sigNames = response.Aggs.SignificantTerms("significant_names");
+sigNames.Should().NotBeNull();
+sigNames.DocCount.Should().BeGreaterThan(0);
+----
+
+[[significant-terms-exact-value-filter]]
+[float]
+== Filtering with exact values
+
+Using significant terms aggregation with filtering to exclude specific values
+
+=== Fluent DSL Example
+
+[source,csharp]
+----
+s => s
+.Aggregations(a => a
+    .SignificantTerms("significant_names", st => st
+        .Field(p => p.Name)
+        .MinimumDocumentCount(10)
+        .MutualInformation(mi => mi
+            .BackgroundIsSuperSet()
+            .IncludeNegatives()
+        )
+        .Exclude(new [] { "pierce" })
+    )
+)
+----
+
+=== Object Initializer Syntax Example
+
+[source,csharp]
+----
+new SearchRequest<Project>
+{
+    Aggregations = new SignificantTermsAggregation("significant_names")
+    {
+        Field = Field<Project>(p => p.Name),
+        MinimumDocumentCount = 10,
+        MutualInformation = new MutualInformationHeuristic
+        {
+            BackgroundIsSuperSet = true,
+            IncludeNegatives = true
+        },
+        ExcludeTerms = new SignificantTermsIncludeExclude(new[] { "pierce" })
+    }
+}
+----
+
+[source,javascript]
+.Example json output
+----
+{
+  "aggs": {
+    "significant_names": {
+      "significant_terms": {
+        "field": "name",
+        "min_doc_count": 10,
+        "mutual_information": {
+          "background_is_superset": true,
+          "include_negatives": true
+        },
+        "exclude": [
+          "pierce"
+        ]
+      }
+    }
+  }
+}
+----
+
+=== Handling Responses
+
+[source,csharp]
+----
+response.ShouldBeValid();
+var sigNames = response.Aggs.SignificantTerms("significant_names");
+sigNames.Should().NotBeNull();
+sigNames.DocCount.Should().BeGreaterThan(0);
+----
+