Fix output extension directive and add test

Fixes #142

Author: mhutch

Mono.TextTemplating.Tests/ProcessingTests.cs

@@ -117,6 +117,22 @@ public async Task SetLangVersionViaAdditionalArgsInProcess ()
 		}
 #endif
 
+		[Fact]
+		public async Task SetOutputExtension ()
+		{
+			string inputContent = "<#@ output extension=\".cfg\" #>";
+			string inputFile = "hello.tt";
+			string outputName = "hello.txt";
+
+			// this reproduces the calls made by dotnet-t4
+			var gen = new TemplateGenerator ();
+			var pt = gen.ParseTemplate (inputFile, inputContent);
+			TemplateSettings settings = TemplatingEngine.GetSettings (gen, pt);
+			(outputName, _) = await gen.ProcessTemplateAsync (pt, inputFile, inputContent, outputName, settings);
+
+			Assert.Equal ("hello.cfg", outputName);
+		}
+
 		[Fact]
 		public async Task ImportReferencesTest ()
 		{

Mono.TextTemplating/Mono.TextTemplating/TemplateGenerator.cs

@@ -268,7 +268,7 @@ public string PreprocessTemplate (
 
 			var outputContent = await Engine.ProcessTemplateAsync (pt, inputContent, settings, this, token).ConfigureAwait (false);
 
-			return (outputFileName, outputContent);
+			return (OutputFile, outputContent);
 		}
 
 		#region Virtual members