Fix null_object double visibility bug.

The error being raised had the word "private" in
it only because the method itself had that in its
name, but this did not indicate it was raising
the normal "private method called" error raised
by ruby.
Allow null object verified doubles to receive private methods using `send`.

This mirrors the behavior of the real object.
Fix 1.8.7.

I'm not sure why, but aliasing `__send__` to `send`
on 1.8.7 doesn't work correctly. Defining the alias
manually does, for some reason.

While we're at it, it makes more sense to define
`__send__` as primary and `send` as secondary,
as it's more likely users will stub `send`
(e.g. for an `email` object).
Add specs to ensure `send` can be stubbed.
Work around JRuby bug.
Merge pull request #524 from rspec/fix-null-object-visibility

Fix null object visibility