added 502 to RETRY_HTTP_CODES
arg_to_iter: treat items the same way as dicts (ie. non iterables). fixes #248
arg_to_iter: replaced double call to isinstance with a single one. refs #248
remove debugging code
Log overriden Scrapy settings when Scrapy starts.

This is useful for debugging, to quickly find out which settings where
used for a specific spider run. dict settings are omitted for brevity.
DEBUG level is used for consistency with "Enabled
extensions/middlewares" lines (which share a similar purpose).