net/http: correctly close fake net.Conns

Fix an inverted test in fakeNetConn.Close that caused closing
a connection to not break the other half of the connection.

Change-Id: I4e53f78402f8e503c749d57f294a4524abdccfb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/722220
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/http/netconn_test.go

@@ -180,9 +180,10 @@ func (c *fakeNetConn) Close() error {
 	c.loc.unlock()
 	// Remote half of the connection reads EOF after reading any remaining data.
 	c.rem.lock()
-	if c.rem.readErr != nil {
+	if c.rem.readErr == nil {
 		c.rem.readErr = io.EOF
 	}
+	c.rem.writeErr = net.ErrClosed
 	c.rem.unlock()
 	if c.autoWait {
 		synctest.Wait()