Only call order when the given association responds to it.

Fixes #966.

This check is necessary because the given association may not respond to `order`.
Add CHANGELOG entry.
Check if the relation responds to `order`, instead of checking the
`reflection`.
Merge pull request #1011 from plataformatec/check_before_calling_order_on_association

Check if the given association respond to `order` before calling it.