disallowing writes only when fully matched request is in cassette

[miki725]

cassette.filter_request only filters some request attributes like headers but does not take into account any of the match_on configurations. As a result if a similar request has been seen before but does not fully match as per match_on, writes will be disallowed. cassette.__contains__ however fully takes into account all match_on matchers and therefore will only guard against recording to the cassette when the same request has been previously recorded

[miki725]

if this looks reasonable I can work on adjusting/adding tests as necessary but would appreciate any feedback first