Binding twice on the same addr returns OK both times on Windows

[blabaere]

Hello,

I wrote a test showing that on Windows, when using std::net::TcpListener, the second bind returns Err but using mio::net::tcp::TcpListener, both calls return OK. On linux, the tests are passing.

The Appveyor build.
The Travis build.
The source code.

Regards,
Benoît