Easier session scoped mock (compatible with nested mocks)

[clslgrnc]

Resolve conflicts between #116 and #110.

[clslgrnc]

b8746ce is necessary to make real_http work as well as interactions between global and session mocks.
Indeed #116 changed call to _original_send, that always takes session as an argument, into a call to self._last_send for which session is implicit when mocking a specific session.
Likewise, restoration of get_adapter was not always done on the right object.

[clslgrnc]

There are still a few issues with nested global and session mocks like in the following example:

    url = 'mock://example.test/'
    with requests_mock.Mocker() as global_mock:
        global_mock.get(url, text='global')
        session = requests.Session()
        with requests_mock.Mocker(session=session) as session_mock:
            session_mock.get(url, real_http=True)
            text = session.get(url).text

What should happen?

• [x] text is "global" since that global mock was in place at session creation.
• [ ] an error since the given url is invalid and global and session mock are independents
• [ ] global and session mocks should not be mixed up and we should update doc

I feel like the first option is the best, but what currently happens is the second one.

---

Edit: last commit solved the issue

[jamielennox]

I did tack a small change on here just to clean it up so we can merge it rather than go back and forward in review. I hope this is ok for you both.

Thanks you @clslgrnc @akeeman