Fixed issue with content type not being set correctly

When 'header' was not used, the default headers were being set incorrectly.  It was setting Content-Type and Access-Control-Allow-Origin headers as a JSON string instead of JSON object.  Also, the header was not getting set with the passed 'contentType', rather it was using 'application/json' always.

Also modified sample app to cover the scenario which caught this bug (POST ajax call without dataType defined + modifying span tag with mocked response text)

Signed-off-by: Daniel Koo <danieljamin.koo@gmail.com>