Make inline SVG fill parent element by default.

Fixes a small hover area bug for **Return** button:

![hover-area-bug](https://cloud.githubusercontent.com/assets/389387/3647315/fc3ba0f2-10fc-11e4-91f8-e175be0a1149.gif)

Just [a quirk in specification](http://www.w3.org/TR/CSS2/visudet.html#inline-replaced-height) and [browser vendors are OK with it](https://bugzilla.mozilla.org/show_bug.cgi?id=736431).