refactor Removing intermediate variable when calling Aria.classDefinition.

Some files are using an intermediate variable before calling
Aria.classDefinition or Aria.tplScriptDefinition.
This is useless, and can prevent some automatic tools from working on
those files.
This commit removes the intermediate variable, which makes Aria Templates
files more consistent.

Close #729.
fix #728 Headers should not be shared between request handler instances

JSONRequestHandler used to have a headers which were implicitly static,
causing issues when modifying those headers with
RequestHandler#getHeaders().
fix #727 Callback failure when submitting a JSON request should be handled in the framework.

Close #731