Add into_inner method to StartHandshake

This allows for recovering the underlying raw connection, even though
it's had the ClientHello read off of it already.

src/lib.rs

@@ -331,6 +331,12 @@ where
             state: TlsState::Stream,
         }))
     }
+
+    /// Returns the original IO instance. Note that at least the ClientHello will have been read
+    /// off the IO by this point.
+    pub fn into_inner(self) -> IO {
+        self.io
+    }
 }
 
 /// Future returned from `TlsConnector::connect` which will resolve