normalize header values to bytes
fix ftp test cases
Merge pull request #828 from dangra/string-headers

normalize header values to bytes (and PY3)
PY3 enable more tests
PY3 fix test_utils_sitemap.

Sitemap class is only initialized with bytes by Scrapy;
broken tests was only a testing issue.
PY3 port scrapy.utils.spider
PY3 enable more tests
Selector: __str__() calls extract()
Selector.__str__() tests