Include buffered data when returning underlying connection (#679)

[ipostelnik]

Fixes #679

When returning underlying connection and there's buffered data, return a wrapper that gives access to buffered data first.

[ghost]

This PR breaks applications that use UnderlyingConn() to access the *net.TCPConn or *tls.Conn directly. An example use is setting the keep alive period.

[ipostelnik]

This is better handled by adding Hijack method that doesn't break existing users of UnderlyingConn