Accept mw.title object as the title argument of frame:expandTemplate()

This commit fixes an error with using a mw.title object referring to a
mainspace page as the title argument to frame:expandTemplate(), by
adding a leading colon to prevent the function from searching in the
Template namespace.

Bug: 47601
Change-Id: I4cdc05571598bf7998f4cf0f2691bf86188c3c5d
Merge "Accept mw.title object as the title argument of frame:expandTemplate()"