Html: added protection against innerHTML mXSS vulnerability [Closes nette/nette#1496] (possible BC break)

IE8 for code `<div attr="``foo=bar">` produces invalid innerHTML `<div attr=``foo=bar>`. Adding a space at the end of the attribute forces IE to put quotes around the attribute.

More info:
http://www.nds.rub.de/research/publications/mXSS-Attacks/
http://www.slideshare.net/x00mario/the-innerhtml-apocalypse