Add code to warn about the should syntax.

Signed-off-by: Sam Phippen <samphippen@googlemail.com>
Add more specs and fix should syntax warning.
Add a changelog entry for #326.
Tweaks for deprecation of `:should` being enabled by default.

- Improve changelog (it's deprecated not disabled).
- Fix indentation.
- Prefer `.` to `::` for message sends.
- Ensure `configure_default_syntax` is called at the
  right time to prevent false-positive tests.
- Group the specs that rely on the default being
  configured so we can put that in a `before` hook.
- No need to configure the default in an `after` hook.
- Don't add helper methods to every object.
Merge branch 'pr/326'