Ruby - Add `has_key?` method

Aliased as `include?`, as well as implements `respond_to_missing?`.
Python - Add `__contains__` method

My initial intention was to implement the `has_key` method, but the
Python 2 docs state

> `has_key()` is deprecated in favor of `key in d`.

Implementing `__contains__` allows the use of `in`, so I'm going with
that over the deprecated approach.

https://docs.python.org/2/library/stdtypes.html?highlight=has_key#dict.has_key
C# - Add `ContainsKey` method

I don't care for the use of exceptions as control flow, but I'll leave
it for now.