Address ruby 2.1 warning.

From https://github.com/ruby/ruby/blob/v2_1_0_rc1/NEWS:

Hash#reject will return plain Hash object in the future versions, that
is the original object's subclass, instance variables, default value,
and taintedness will be no longer copied, so now warnings are emitted
when called with such Hash.

Longer term I'd like to move away from subclassing Hash
entirely, but this works for now to solve the warnings.