Add tests for X-OC-MTime header handling

This commit is based on the commits from pull request 28066 (included in
018d45cad97e0) from ownCloud by Artur Neumann and Phil Davis.

Unit tests are currently run only on systems that support negative
mtimes, so no special handling of negative values was included in the
tests to keep the test code more manageable.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>