Move token retrieval outside of initializer method
Move token retrieval outside of initializer method

I don't believe Warden guarantees the `request` method during strategy
initialization, so I've moved it to the `valid?` method for now. It may be
necessary to move it to the actual `authenticate!` call where it is definitely
guaranteed to exist and be accurate.

edmondchui/warden-doorkeeper,kolorahl/warden-doorkeeper