fix test and a crash when closed

[artemredkin]

Fixes a crash in testUncleanCloseThrows.

Motivation:
This PR aims to fix two issues:

1. A crash on precondition, that was not removed when I re-factored how shutdowns work, it is absolutely normal to get a connection failure when client is shutting down, for example in the following scenario:

• send a request that will fail to connect
• shutdown the client
• when connect error will be processed, client will be .closed, so it's a completely normal situatuon

2. A racy test, that test tries to establish a connection and shutdown the client while this connection is alive. But it does so in a racy fashion, there is no guarantee that when we get to client shutdown, connection has been established. This will be addressed by using NIOHTTP1TestServer, where we wait for .head before we attempt to shutdown the client.

Modifications:

1. Rewritten the testUncleanCloseThrows to use NIOHTTP1TestServer to eliminate a race
2. Removed precondition in ConnectionState.swift
3. Added a test to make sure that connectFailed is working as expected when the client is .closed

Result:
Closes #248

[weissi]

can't we achieve this state by driving the state machine rather than forcing in a possibly illegal state?

[artemredkin]

ah, good catch, we can indeed, done, thanks! (also fixed a similar test above)

[weissi]

Awesome, thanks, LGTM!