CRM-15674 Content type for json results should be application/json

According to http://stackoverflow.com/questions/477816/what-is-the-correct-json-content-type (which refers to http://www.ietf.org/rfc/rfc4627.txt), the content type for json is application/json. The API serves it as text/javascript.

This is probably not very critical. But it seems that some development frameworks that expect json results don't like text/javascript.