Added `base_url` property to Response class.
Better `Crawled ...` message in Engine.
Removed rfc advice, to use 'iso-8859-1' encoding as default in
content-type.
Added more unittests to http_compression pipeline mw.
Corrected encoding issues in LxmlLinkExtractor.
Added `quiet` attribute to RequestDropped exception, not be logged when
occurred.
Added `DuplicateFilter` pipeline mw.
Don't push into queues in Engine, when engine is stopped.