[Messages] Keyboard should always close when send button is pressed . r = gnarf

https://bugzilla.mozilla.org/show_bug.cgi?id=906581
  This is a regression issue.The problem is with the assimilateRecipients() in thread_ui.js
  Here we are always refocusing with the input, refocus: this.input.
  When we are sending a message,we should not keep the focus in the input as the refocus happens  asynchronously.

 Please review the patch