Created methods to append and finish body content on Request

[vltr]

Hi! I just created two methods to append and finish the body content "feed" on request.py so the underlying body instance can have certain flexibility (example: one could use BytesIO instead of a list for storing the body data if a custom Request-like class is provided to the Sanic constructor).

I also modified server.py to reflect the changes on the Request class.

If any documentation is required regarding this, please let me know. Also, I'm not sure if this change is the best approach to the Request.body attribute so I'm open to suggestions 😉

[seemethere]

@vltr would you mind rebasing your PR?

[vltr]

@seemethere yeah, no problem. In fact, one problem: unknown repository. How did I managed to do that? 😱

If I don't figure out if there's a way to fix this, I'll just make another PR replacing this one.

[vltr]

I was able to get the code from the PR, rebase and etc; but unknown repository PRs are readonly, so it is not possible to append directly 😒

So, I made a new PR #1379 to replace this one. Sorry for the mess.