When doing the functional tests use the new function
`projects.transfer` instead of the deprecated function
`projects.transfer_project()`

utils.EncodedId() has basically the same functionalityy of using
utils._url_encode(). So remove utils._url_encode() as we don't need
it.

Add EncodedId string class. This class returns a URL-encoded string
but ensures it will only URL-encode it once even if recursively
called.

Also added some functional tests of 'lazy' objects to make sure they
work.

Make sure all usage of the ID in the URL path is encoded. Normally it
isn't an issue as most IDs are integers or strings which don't contain
a slash ('/'). But when the ID is a string with a slash character it
will break things.

Add a test case that shows this fixes wikis issue with subpages which
use the slash character.

Closes: #1079

Commit 8da0b758c589f608a6ae4eeb74b3f306609ba36d added the `lazy`
parameter to the services `get()` method but missed then using the
`lazy` parameter when it called `super(...).get(...)`

Closes: #1828

We were using `str.replace()` calls to take care of URL encoding
issues.

Switch them to use our `utils._url_encode()` function which itself uses
`urllib.parse.quote()`

Closes: #1356

The arguments `should_remove_source_branch` and
`merge_when_pipeline_succeeds` are optional arguments. We should not
be setting any default value for them.

https://docs.gitlab.com/ee/api/merge_requests.html#accept-mr

Closes: #1750

Add a functional test of test mergerequest.get() and
mergerequest.get(..., lazy=True)

Closes: #1425