Improve perf by using Hashie::Mash#[]

Hashie::Mash#method_missing is really slow, and it's called many times.
Running `rake perf:ips` from https://github.com/intridea/omniauth/pull/775
reveals a 60% performance boost.

Before:

  Calculating -------------------------------------
                   ips       649 i/100ms
  -------------------------------------------------
                   ips     6757.6 (±6.3%) i/s -      33748 in   5.013527s

After:

  Calculating -------------------------------------
                   ips      1028 i/100ms
  -------------------------------------------------
                   ips    10738.5 (±7.0%) i/s -      53456 in   5.001545s