HTTP::Server bug on keep alive request

[asterite]

The problem is in these lines:

crystal/src/http/server.cr

Lines 171 to 176 in fd9bd6f

	begin
	request = HTTP::Request.from_io(io)
	rescue e
	STDERR.puts "Bug: Unhandled exception while parsing request"
	e.inspect_with_backtrace(STDERR)
	end

On the rescue branch request must be set to nil, otherwise on an exception it will keep the value of the previous iteration, which is wrong.

The fix is easy, but I'd like to add a spec to prevent regressions... but I don't know how to easily do it.

Originally discovered by @mverzilli and @waj

[jhass]

We probably should allow injecting the server socket instead of forcing it to be build by HTTP::Server itself. Then the tests can just inject a fake backed by MemoryIO or similar. That would also partly address #2735

[asterite]

There's another issue: when the IO is closed the server tries to response with an error, many times, but the IO is closed. This ends up bubbling an exception to main.