we have to use a different mime type to separate js from jsonp

however this also requires https://github.com/willdurand/Negotiation/pull/8
keep looping over the rules unless fallack_format is set to false
Merge pull request #558 from FriendsOfSymfony/jsonp_fixes

use a different mime type to separate js from jsonp
Merge pull request #557 from FriendsOfSymfony/format_tweaks

keep looping over the rules unless fallack_format is set to false