Added fix for #131 by changing assignment of request headers

Instead of being placed on the `mockHandler` object they are now placed on the `requestSettings` object. The test for these has to use the `$.mockjax(function(requestSettings) { ... })` format otherwise there is no access to the _request_ headers (the `response` method has access to `origSettings`, which does not get headers placed using `xhr.setRequestHeader()`).

I also included a minor fix for the mockjax settings to be reset after each test (if we set these in a test, it could affect other tests) and some linting cleanup.