StreamGenerator: Fix output encoding

Re-applies https://github.com/rdio/vernacular/commit/15e769b267 to fix
output encoding. In particular this fixes generating UTF-16 strings files
for iOS.

Vernacular.Tool/Vernacular.Generators/StreamGenerator.cs

@@ -59,14 +59,8 @@ namespace Vernacular.Generators
 
         protected sealed class GeneratorWriter : StreamWriter
         {
-            private Encoding encoding;
-            public override Encoding Encoding {
-                get { return encoding; }
-            }
-
             public GeneratorWriter (Stream stream, Encoding encoding)
-                : base (stream) {
-                this.encoding = encoding;
+                : base (stream, encoding) {
                 NewLine = "\n";
             }
         }