simplifies OmniAuth::Strategy.default_options

I noticed that OmniAuth::Strategy::Options inherits from Hashie::Mash, meaning that it will initialize even if what you pass in is nil. Then, memoizing default_options with the ||= syntax achieves the same result as the return. 

I would greatly appreciate any feedback.

Thanks :)
Rae

Not sure about relying on the fact that `if` in modifier position returns `nil` when the conditional is `false` but :+1: for memoizing `default_options` with `||=`.