Do not return highest priority format immediately when catch-all is detected

Related to issue 372

From the HTTP/1.1 spec (http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html):

Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. For example,

       Accept: text/*, text/html, text/html;level=1, */*
have the following precedence:

       1) text/html;level=1
       2) text/html
       3) text/*
       4) */*
Update FormatNegotiator.php

Fixed undefined variable issue.
Merge pull request #15 from teohhanhui/patch-1

Do not return highest priority format immediately when catch-all is dete...