fix($http): don't parse templates as JSON

The default behaviour causes issues with a wide variety of custom
interpolation markers. Even if the JSON test regexps were strengthened to
closer match the standard JSON format, it would still limit the possibilities
of different custom interpolation markers.

BREAKING CHANGE: $http calls using either the $templateCache, or with custom 
Accept headers not including `application/json` will no longer parse JSON.

Closes #5756