MDL-48685 tests: Rewrite useragent tests
MDL-48685 tests: Add additional User Agents

* Firefox 33; Mac OS X 10.10
* Chrome 39; Mac OS X
* Safari; iPhone 6 Plus; iOS 8.1; Build 12B411
* Opera 12.15 (Build 1748); Mac OS X
MDL-48685 tests: Add testing for supports_json_contenttype
MDL-48685 core_useragent: Correct IE 5.0 issues