Remove extra charset parameter if present

[jfparadis]

Caja fails to load resources when the server returns the charset in the content type header.

This header works:

content-type: application/javascript

But this fails without the fix:

content-type: application/javascript; charset=utf-8

In that case, Caja throws an error:

Unimplemented content-type application/javascript; charset=utf-8

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