Set encoding to UTF8 without BOM (#245)

`Encoding.UTF8` will include a UTF8 BOM character, which is invalid according to the [JSON spec RFC7159, Section 8.1](https://tools.ietf.org/html/rfc7159#section-8.1).
This was causing issues with (in particular) JavaScript clients that call `JSON.parse` on the response.
`UTF8Encoding` allows us to specify whether to include the BOM character or not.

Saule/Http/JsonApiMediaTypeFormatter.cs

@@ -155,10 +155,10 @@ public override MediaTypeFormatter GetPerRequestFormatterInstance(
 
         private async Task WriteJsonToStream(JToken json, Stream stream)
         {
-            using (var writer = new StreamWriter(stream, Encoding.UTF8, 2048, true))
+            using (var writer = new StreamWriter(stream, new UTF8Encoding(encoderShouldEmitUTF8Identifier: false), 2048, true))
             {
                 await writer.WriteAsync(json.ToString(Formatting.None, _config.JsonConverters.ToArray()));
             }
         }
     }
-}
+}