Modularize NullColorizer

Other colorizers s.t. ConsoleCodes are module with module_functions.
NullColorizer was a class and the class does not respond to `wrap()`. This
patch fixes the mismatch.

I approached a problem about NullColorizer in PR #1637, but the fix was not complete.
I am sorry about my carelessness.
I intended in  PR #1637 to invoke [line 237](https://github.com/tomykaira/rspec-core/blob/modularize_NullColorizer/lib/rspec/core/notifications.rb#L237) to assert `colorizer.wrap` works.
However, the test case added in the PR was insufficient, and the line was not covered. I am ashamed of this mistake.
In order to cover the line, the example group must be a shared example. So I added metadata: `{:shared_group_name => 'double shared group'}`.
Because of this, [line 246](https://github.com/tomykaira/rspec-core/blob/87a2a9f0301fb651e54632fd48e680dec7766a8c/lib/rspec/core/notifications.rb#L246) became to be called.
Here, the `location` is required.

Does this explanation answer your question? Please point out if I misunderstand.