fix pcl build error

Author: mythz

src/ServiceStack.Text/ReflectionExtensions.cs

@@ -1796,13 +1796,17 @@ public static Type GetCollectionType(this Type type)
 
         static Dictionary<string, Type> GenericTypeCache = new Dictionary<string, Type>();
 
-        public static Type GetCachedGenericType(this Type type, Type[] argTypes)
+        public static Type GetCachedGenericType(this Type type, params Type[] argTypes)
         {
-            if (!type.IsGenericTypeDefinition)
+            if (!type.IsGenericTypeDefinition())
                 throw new ArgumentException(type.FullName + " is not a Generic Type Definition");
 
+            if (argTypes == null)
+                argTypes = Type.EmptyTypes;
+
             var sb = StringBuilderThreadStatic.Allocate()
                 .Append(type.FullName);
+
             foreach (var argType in argTypes)
             {
                 sb.Append('|')

src/ServiceStack.Text/TypeConstants.cs

@@ -13,7 +13,7 @@ public static class TypeConstants
         public static readonly bool[] EmptyBoolArray = new bool[0];
         public static readonly byte[] EmptyByteArray = new byte[0];
         public static readonly object[] EmptyObjectArray = new object[0];
-        public static readonly Type[] EmptyTypeArray = new Type[0];
+        public static readonly Type[] EmptyTypeArray = Type.EmptyTypes;
         public static readonly FieldInfo[] EmptyFieldInfoArray = new FieldInfo[0];
         public static readonly PropertyInfo[] EmptyPropertyInfoArray = new PropertyInfo[0];