Fix: Tests failing in Firefox (Issue #14)

Issue #14 fixed.

The reason being that the userAgent property is readonly [1], per spec [2].

[1] http://mxr.mozilla.org/mozilla-central/source/dom/webidl/Navigator.webidl#45
[2] http://www.whatwg.org/specs/web-apps/current-work/#the-navigator-object

Instead of changing navigator.userAgent of browser, the userAgent parameter of Modernizr.Detectizr.device changed at the initialization stage.