Fix a few windows-specific issues

This commit fixes longstanding issues with the HTTP client used to communicate with Grid servers. I was creating a generic client with no defined timeouts and infinite retries. This works fine when everything is normal, but it can lead to execution deadlocks whenever things are in a bad state.

I also fixed an issue interacting with the Appium Windows engine, which throws a generic WebDriverException instead of UnsupportedCommandException in response to driver.switchto().defaultContent().