Dupefilter request_fingerprint test and docs

This PR is related to #533, title is pretty self-explanatory, changes are minor. Please check test and grammar.

One more thing I was thinking about - maybe it would be better to rename `request_fingerprint` method to `get_request_fingerprint`? Can name clash between `RFPDupeFilter` method and `scrapy.utils.request.request_fingerprint` function cause confusion to people? What do you think?