Added missing Similarity method to StringMappingDescriptor

Author: gmarz

src/Nest/Domain/Mapping/Descriptors/StringMappingDescriptor.cs

@@ -18,6 +18,12 @@ public StringMappingDescriptor<T> Name(Expression<Func<T, object>> objectPath)
 		{
 			this._Mapping.Name = objectPath;
 			return this;
+		}
+
+		public StringMappingDescriptor<T> Similarity(string similarity)
+		{
+			this._Mapping.Similarity = similarity;
+			return this;
 		}
 
 		public StringMappingDescriptor<T> IndexName(string indexName)

src/Tests/Nest.Tests.Unit/Core/Map/FluentMappingFullExampleTests.cs

@@ -126,6 +126,7 @@ public void MapFluentFull()
 				.Properties(props => props
 					.String(s => s
 						.Name(p => p.Name)
+						.Similarity("mysimilarity")
 						.IndexName("my_crazy_name_i_want_in_lucene")
 						.IncludeInAll()
 						.Index(FieldIndexOption.analyzed)

src/Tests/Nest.Tests.Unit/Core/Map/Properties/PropertiesTests.cs

@@ -15,7 +15,8 @@ public void StringProperty()
 			var result = this._client.Map<ElasticsearchProject>(m => m
 				.Properties(props => props
 					.String(s => s
-						.Name(p => p.Name)
+						.Name(p => p.Name)
+						.Similarity("mysimilarity")
 						.IndexName("my_crazy_name_i_want_in_lucene")
 						.IncludeInAll()
 						.Index(FieldIndexOption.analyzed)

src/Tests/Nest.Tests.Unit/Core/Map/Properties/StringProperty.json

@@ -3,6 +3,7 @@
 		"properties": {
 			"name": {
 				"type": "string",
+				"similarity": "mysimilarity",
 				"index_name": "my_crazy_name_i_want_in_lucene",
 				"store": "yes",
 				"index": "analyzed",