Make predefined stopwords support backwards compatible

Author: gmarz

src/Nest/Domain/Analysis/TokenFilter/StopTokenFilter.cs

@@ -13,7 +13,24 @@ public StopTokenFilter() : base("stop")
 
         }
 
-        [JsonProperty("enable_position_increments")]
+		[JsonProperty("stopwords")]
+		internal object _Stopwords { get; set; }
+
+		[JsonIgnore]
+		public IEnumerable<string> Stopwords
+		{
+			get { return _Stopwords as IEnumerable<string>; }
+			set { _Stopwords = value; }
+		}
+
+		[JsonIgnore]
+		public string PredefinedStopwords
+		{
+			get { return _Stopwords as string; }
+			set { _Stopwords = value; }
+		}
+
+		[JsonProperty("enable_position_increments")]
 		public bool? EnablePositionIncrements { get; set; }
 
         [JsonProperty("ignore_case")]
@@ -22,9 +39,6 @@ public StopTokenFilter() : base("stop")
         [JsonProperty("stopwords_path")]
         public string StopwordsPath { get; set; }
 
-        [JsonProperty("stopwords")]
-        public object Stopwords { get; set; }
-
 		[JsonProperty("remove_trailing")]
 		public bool? RemoveTrailing { get; set; }
     }

src/Tests/Nest.Tests.Unit/Core/Indices/Analysis/TokenFilters/TokenFilterTests.cs

@@ -33,7 +33,7 @@ public void TestStopTokenFilterWithPredefinedLanguageList()
 				.TokenFilters(t => t
 					.Add("my_stop", new StopTokenFilter
 					{
-						Stopwords = "_english_"
+						PredefinedStopwords = "_english_"
 					})
 				)
 			);