Fix: Do not expect response for initiatorType Ping requests

[MC-Squared]

Motivation

HTML supports adding a ping attribute to <a> tags. In such cases, the browser sents a POST request to the set URL(s) before navigating to the new URL (See w3schools doc here and Mozilla docs on initiatorType which includes ping as an option).
Unfortunately, because these requests do not receive a server response Ferrum adds them to the list of pending_connections, meaning that subsequent calls to wait_for_idle will timeout waiting for these never-will-complete requests.

Changes

• Add Request#response_expected? method to indicate if the request expects a response from the server (currently 'ping' is the only request I know of that would not get a response)
• Add Exchange#response_expected? that calls the request method. Note, if request.nil? we default to true to match the current behaviour
• Update Exchange#finished? so that any requests that do not expect responses are now also considered 'finished'

[route]

Could you add a proper test with tag?

[route]

ping

[MC-Squared]

Updated - I have guessed a bit on where best to locate the new specs so if they need to be moved/re-arranged just let me know

[route]

Thanks! Merging, though I have some patch to apply on it after merging.