Ensure the Locator always takes a SimplePie_File
Ensure the Locator always takes a SimplePie_File

Before, any arbitrary object could be passed in. Now, it *must* be a
subclass of SimplePie_File to ensure that we have a sane interface.

mblaney/simplepie,songfj/simplepie,Alkarex/simplepie,barryvdh/simplepie,Alkarex/simplepie,Alkarex/simplepie,blat/simplepie,songfj/simplepie,ropik/simplepie,blat/simplepie,simplepie/simplepie,FreshRSS/simplepie,simplepie/simplepie,ropik/simplepie,blat/simplepie,wikidi/simplepie,skyzyx/simplepie,barryvdh/simplepie,wikidi/simplepie,skyzyx/simplepie,barryvdh/simplepie,songfj/simplepie,FreshRSS/simplepie,mblaney/simplepie,wikidi/simplepie