res_http_media_cache: Cleanup audio format lookup in HTTP requests

Asterisk first looks at the end of the URL to determine the file
extension of the returned audio, which in many cases will not work
because the URL may end with a query string or a URL fragment. If that
fails, Asterisk then looks at the Content-Type header and then finally
parses the URL to get the extension.

The order has been changed such that we look at the Content-Type
header first, followed by looking for the extension of the parsed
URL. We no longer look at the end of the URL, which was error prone.

ASTERISK-29527 #close

Change-Id: I1e3f83b339ef2b80661704717c23568536511032