[client] panic when sending large body

[joelwurtz]

I got case where the client send a very large body, when it use http2 it panics because there is no capacity left :

thread 'agent' panicked at /home/joelwurtz/.cargo/git/checkouts/xitca-web-8aaa2e840e2a79ba/479e12c/client/src/h2/proto/dispatcher.rs:108:22:
No capacity left. http2 request is dropped

I do believe this is an expected error, and i should adapt the windows size for this request, but there is no way to specify that actually.

Also i do believe this should return a proper error instead of a panic.

Can do the PR just need to be sure on what is best here.