Move the 'gitlab/tests/' directory to 'tests/unit/' so we have all the
tests located under the 'tests/' directory.

The function sanitize_parameters() was used when the v3 API was in
use. Since v3 API support has been removed there are no more users of
this function.

Git refnames are relatively free-form and can contain all sort for
special characters, not just `/` and `#`, see
http://git-scm.com/docs/git-check-ref-format

In particular, Debian's DEP-14 standard for storing packaging in git
repositories mandates the use of the `%` character in tags in some
cases like `debian/2%2.6-21`.

Unfortunately python-gitlab currently only escapes `/` to `%2F` and in
some cases `#` to `%23`. This means that when using the commit API to
retrieve information about the `debian/2%2.6-21` tag only the slash is
escaped before being inserted in the URL path and the `%` is left
untouched, resulting in something like
`/api/v4/projects/123/repository/commits/debian%2F2%2.6-21`. When
urllib3 seees that it detects the invalid `%` escape and then urlencodes
the whole string, resulting in
`/api/v4/projects/123/repository/commits/debian%252F2%252.6-21`, where
the original `/` got escaped twice and produced `%252F`.

To avoid the issue, fully urlencode identifiers and parameters to avoid
the urllib3 auto-escaping in all cases.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

Refactor a bit to handle this change, and add unit tests.

Closes #779