Errors have not been detected by broken travis runs.

`all` kwarg is used to manage GitlabList generator behaviour.
However, as it is not poped from kwargs, it is sent to Gitlab API.
Some endpoints such as [the project commits](https://docs.gitlab.com/ee/api/commits.html#list-repository-commits) one,
support a `all` attribute.
This means a call like `project.commits.list(all=True, ref_name='master')`
won't return all the master commits as one might expect but all the
repository's commits.
To prevent confusion, the same kwarg shouldn't be used for 2 distinct
purposes.
Moreover according to [the documentation](https://python-gitlab.readthedocs.io/en/stable/gl_objects/commits.html#examples),
the `all` project commits API endpoint attribute doesn't seem supported.

fix(api): Don't try to parse raw downloads

http_get always tries to interpret the retrieved data if the
content-type is json. In some cases (artifact download for instance)
this is not the expected behavior.

This patch changes http_get and download methods to always get the raw
data without parsing.

Closes #683

Provide another way to send data to gitlab with a new `query_parameters`
argument. This parameter can be used to explicitly define the dict of
items to send to the server, so that **kwargs are only used to specify
python-gitlab specific parameters.

Closes #566
Closes #629

When requests are throttled (HTTP response code 429), python-gitlab
assumed that 'Retry-After' existed in the response headers. This is
not always the case and so the request fails due to a KeyError. The
change in this commit adds a rudimentary exponential backoff to the
'http_request' method, which defaults to 10 retries but can be set
to -1 to retry without bound.