[#1855] Fix AIOOB exception in WS.getQueryString

A minimal fix for the exception in getQueryString without breaking any backwards compatibility. Like I mention in [lighthouse #1855](http://play.lighthouseapp.com/projects/57987-play-framework/tickets/1855-aioob-exception-in-wsgetquerystring), I think a much better solution would be to remove this method and fix OAuth2.java, but that might break outside code.