Fixes #1546 - link helper adding duplicate protocol. props jkv1

The URL text field needs to be trimmed of whitespace since the user
could paste text that has spaces in it.

http://ios.trac.wordpress.org/ticket/1546

git-svn-id: http://ios.svn.wordpress.org/trunk@4009 7d847f7c-ffe1-4a24-85be-b788d016d96a