Add sane way to close() pending request

[clue]

Currently, calling close() on a pending Request stream does not necessarily close all underlying resources.

Also, it should probably cancel a pending connection attempt, so this likely depends on reactphp-legacy/socket-client#40

[clue]

Calling close() should stop connecting / sending as expected and then close the stream, see also #70 for more details about improper event handling.

This is something I'll look into shortly