selenium: Support W3C WebDriver Set Timeouts format

From what I can tell, all Selenium client bindings apart from C# use an incorrect data structure as a body when setting timeout durations:

{
  type: "implicit",
  ms: 42,
}

• Node.js: https://github.com/SeleniumHQ/selenium/blob/master/javascript/node/selenium-webdriver/lib/webdriver.js#L1469
• Ruby: https://github.com/SeleniumHQ/selenium/blob/master/rb/lib/selenium/webdriver/remote/w3c_bridge.rb#L138
• Java: https://github.com/SeleniumHQ/selenium/blob/master/java/client/src/org/openqa/selenium/remote/RemoteWebDriver.java#L834
• Python: https://github.com/SeleniumHQ/selenium/blob/master/py/selenium/webdriver/remote/webdriver.py#L676

The Set Timeouts command expects a data format like this:

{
  implicit: 42,
}

New in W3C WebDriver is that multiple timeout durations can be set at the same time:

{
  implicit: 123,
  pageLoad: 456,
  script: 789,
}

Note that driver implementations will enumerate the own properties of the dictionary, so an unknown field is likely to error. For example, if a client passes {foo: 1, implicit: 2}, this is considered an error scenario by the driver.

See https://github.com/mozilla/geckodriver/issues/511 filed on geckodriver.