Merge pull request #1019 from zzoe2346

* gh-1019:
  Remove duplicate identifier null check in RestDocumentationGenerator

Closes gh-1019

Author: wilkinsona

spring-restdocs-core/src/main/java/org/springframework/restdocs/generate/RestDocumentationGenerator.java

@@ -161,7 +161,6 @@ public RestDocumentationGenerator(String identifier, RequestConverter<REQ> reque
 		Assert.notNull(identifier, "identifier must be non-null");
 		Assert.notNull(requestConverter, "requestConverter must be non-null");
 		Assert.notNull(responseConverter, "responseConverter must be non-null");
-		Assert.notNull(identifier, "identifier must be non-null");
 		Assert.notNull(requestPreprocessor, "requestPreprocessor must be non-null");
 		Assert.notNull(responsePreprocessor, "responsePreprocessor must be non-null");
 		Assert.notNull(snippets, "snippets must be non-null");