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

[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 example_group is not longer supported #2232).
• 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.

[JonRowe]

LGTM!