IIS Mime type for Javascript is "application/x-javascript" not "application/javascript".

The IIS Mime type for javascript does not match either of the two listed
in Zepto.
This causes requests for javascript files to be rejected with a "406 Not
Acceptable".
As much as IIS should use the more standard type, developers will not
necessarily have control over the server.
Add test for loading Javascript and Mime type.

Added tests cases for loading javascript using Ajax and also to check
that the Accept header is being set to the correct list of Javascript
Mime types.
Merge pull request #825 from sgtwilko/Ajax-Mime-Type

IIS Mime type for Javascript is "application/x-javascript" not "application/javascript".