Serving + Closing in 2 separate routines is racy

[alapidas]

Running ListenAndServe() and Close() in 2 separate routines causes the race detector to pick up a race condition, based on the fact that defaultServer has no synchronization protection around it.

Fixed by #37

Worth noting, this change would make Close() block until ListenAndServe is actually called

[lionelbarrow]

This seems unlikely to cause real problems in production, but if it's causing the race detector to fire then 👍 .

[gliptak]

After #37 merged, does this still trigger the race detector?