Handle empty response.body from superagent

Superagent does not always produce a `body`. See
http://visionmedia.github.io/superagent/ for details. When it
doesn't, we should deserialize the raw `response.text` rather
than returning `null`. Currently, the JS client always returns
`null` when the return type is String! This commit fixes
that.

samples/client/petstore/javascript/src/ApiClient.js

@@ -211,7 +211,9 @@
     // See http://visionmedia.github.io/superagent/#parsing-response-bodies
     var data = response.body;
     if (data == null) {
-      return null;
+      // Superagent does not always produce a body; use the unparsed response
+      // as a fallback
+      data = response.text;
     }
     return ApiClient.convertToType(data, returnType);
   };