Check window sizes in HTTP/2

[ericmj]

If you do not check the window sizes you can get errors if the client is sending faster than the server can receive:

Mint.HTTPError: the given data exceeds the connection window size, which is 65535.
The server will refill the window size of the connection when ready.
This will be handled transparently by stream/2.

Use the function Mint.HTTP2.get_window_size/2 https://hexdocs.pm/mint/Mint.HTTP2.html#get_window_size/2 to get the window sizes for the connection and request.

[ericmj]

I see that you are using Mint.HTTP2.request/5 to send the full request body. We recently added a note that we do not recommend doing that: https://hexdocs.pm/mint/Mint.HTTP2.html#request/5-request-body-size and that you should instead use Mint.HTTP2.stream_request_body/3 to send the body.

[keathley]

Thanks @ericmj. I'll take a look at both of these issues.