Avoid raising ContextMiss when testing for key presence.

Raising ContextMiss can be expensive.  The error message includes `@stack.inspect`, which depending on the size and content of the stack can be very costly to generate.  Defaulting to false for has_key? context misses retains the same behaviour without incurring this cost penalty.
Merge pull request #137 from tomafro/master

Avoid raising ContextMiss when testing for key presence.