Fix filtering based on `:example_group` to work for example groups.

Created by: myronmarston

This regressed in #1749. The changes there affected code like this:

RSpec.configure do |c|
  c.include Mod, :example_group => { :file_path => /./ }
end

In RSpec 2, and in RSpec 3 before #1749, this snippet would cause Mod to be included in every example group (since /./ matches all file paths). After #1749, it would not be included in any example groups. Instead, it was included in the singleton class of every example. In practice, this usually worked OK, as the methods from Mod were available to be called very every example, but it had subtle problems:

• If the same method was defined in multiple included modules, the version of the method defined in a module included in this fashion would always get precedence due to the singleton class getting "first dibs" during method dispatch over the example group class. (This is the broken example given in #2232 (closed)).
• I believe this broke c.extend Mod, :example_group => {...} since we don't extend onto singleton classes.

The reason is that the optimizations included in #1749 considered only metadata keys that the groups and examples have in their metadata hashes. In a group hash, :example_group is not a "real" metadata key--instead it is created lazily on first access using a default_proc. But example hashes still have it (it is a reference to the metadata of their group), so it allowed example filtering to work correctly, which in turn hid the bug.

Fixes #2232 (closed).