Add a timeout to RemoteAddr() to allow http.Serve in go < 1.6 work

[keymon]

What

This PR solves the issue described in #1, also when running go < 1.6.

As a summary: go < 1.6, http.Serve() will call from the main go routine RemoteAddr() which blocks the caller until the client sends some data. Because that, http.Serve() will stop accepting new connections.

Motivation

Cloud Foundry gorouter, which is a critical component of this PaaS, has recently merged a PR to add Proxy Protocol support using this library.

But gorouter uses golang 1.5 and it is affected by the issue described in #1 as described in this comment.

I will take a while to get the CF release start using golang 1.6 for the gorouter, and this is a unexpected behaviour using this library, so we consider convenient solve the problem in this library.

Proposed solution

When using Proxy Procotol, the Proxy Protocol will be sent immediately by the proxy once the connection is stablish. Because that, we add an optional timeout to get the Proxy header when calling RemoteAddr(). If we don't get the header in the given time, consider it as a connection that does not use Proxy Protocol.

Consequences and backward compatibility

• This PR maintains the same behaviour than before if Listener.ProxyHeaderTimeout is not defined (zero).
• The Listener struct now has a new field ProxyHeaderTimeout, so you must explicitly name the Listener field when initialising. README has been updated.

Additional changes

We found out that in the case of getting an invalid header, the connection is not closed, despite the comment and the tests expects so. We added the conn.Close().

Additionally, the test was randomly failing as the error returned by Read() might be err.EOF or `syscall.ECONNRESET .

[keymon]

Note about using SetReadDeadLine:

From https://groups.google.com/forum/#!topic/golang-nuts/afgEYsoV8j0

But brief summary: yes, Deadline is a fixed point in time. Set it before a time-sensitive operation (whether that's some composite operation like reading an HTTP header or some low-level Read) and keep calling SetDeadline in the future when you need to extend the deadline. Or reset it with SetDeadline(time.Time{}), which was just fixed.

[crhino]

Any updates on this PR? We are thinking about how to go about patching GoRouter for this and would like to just pull in a new SHA for this library.

[armon]

LGTM! Thanks!