Fix patching of is_connection_dropped from urllib3

With old patching method, urllib3 never detected TCP connections that
were closed by the server side. For example, persistent HTTP connection
that were closed by the server (e.g., due to timeout) were not
recognized as closed. Any following requests that attempted to reuse
the same, closed connection caused the following failure:

    urllib3.exceptions.ProtocolError: ('Connection aborted.',
       RemoteDisconnected('Remote end closed connection without response'
    ))

Fixes: kevin1024#468

vcr/patch.py

@@ -253,6 +253,24 @@ def patched_new_conn(pool):
 
         return patched_new_conn
 
+    def _patched_is_connection_dropped(self, cpool):
+        from .stubs import VCRFakeSocket
+        is_connection_dropped = cpool.is_connection_dropped
+
+        @functools.wraps(is_connection_dropped)
+        def patched_is_connection_dropped(conn):
+            sock = getattr(conn, "sock", None)
+            if isinstance(sock, VCRFakeSocket):
+                # Handle VCRFakeSocket instances here to avoid bad select() calls on Windows
+                # See https://github.com/kevin1024/vcrpy/issues/116
+                return False
+
+            # Let actual is_connection_dropped function handle real sockets.
+            # This allows urllib3 to detect closed TCP connections.
+            return is_connection_dropped(conn)
+
+        return patched_is_connection_dropped
+
     def _urllib3(self):
         try:
             import urllib3.connectionpool as cpool
@@ -349,7 +367,7 @@ def _urllib3_patchers(self, cpool, stubs):
             (cpool, "VerifiedHTTPSConnection", stubs.VCRRequestsHTTPSConnection),
             (cpool, "HTTPConnection", stubs.VCRRequestsHTTPConnection),
             (cpool, "HTTPSConnection", stubs.VCRRequestsHTTPSConnection),
-            (cpool, "is_connection_dropped", mock.Mock(return_value=False)),  # Needed on Windows only
+            (cpool, "is_connection_dropped", self._patched_is_connection_dropped(cpool)),
             (cpool.HTTPConnectionPool, "ConnectionCls", stubs.VCRRequestsHTTPConnection),
             (cpool.HTTPSConnectionPool, "ConnectionCls", stubs.VCRRequestsHTTPSConnection),
         )