jpeg header check fix https://haxenme.atlassian.net/browse/NME-153
allow override default Content-Type in URLLoader
fix MIME content type in URLRequest http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/net/URLRequest.html#contentType
Merge branch 'refs/heads/master' into html5fixes

Conflicts:
	browser/net/URLRequest.hx
Merge remote-tracking branch 'refs/remotes/deep/master' into html5fixes
Merge pull request #55 from profelis/html5fixes

[html5] small fixes [double]