Start deserializing text/javascript without a warning

text/javascript is used by at least the Facebook API. By mentioning it here, we can be "liberal in what we accept, and strict in what we produce.
Correct docs. HTTP::Tiny is not a hard-coded user agent, so don't hard-code here

Also, made the surrounding sentences ready more naturally.
Merge pull request #23 from markstos-/master

Start deserializing text/javascript without a warning