WIP Fix redirection bug.

[rmax]

Scrapy's request meta holds native string as keys. This caused to a
redirected URL being added as seed again.

[sibiryakov]

@rolando I think it's time to remove _request_is_redirected() method and it's call in enqueue_request(). Can you do that? If not, I could fix that.

[rmax]

@sibiryakov How does it look now? I don't know what was the rationale behind adding non-redirected URLs as seeds.

Let me know if it's OK and then I can squash the changes and remove the WIP status.

[codecov-io]

Codecov Report

Merging #261 into master will decrease coverage by -0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
- Coverage   70.15%   70.11%   -0.05%     
==========================================
  Files          68       68              
  Lines        4715     4715              
  Branches      632      576      -56     
==========================================
- Hits         3308     3306       -2     
- Misses       1267     1271       +4     
+ Partials      140      138       -2

Impacted Files	Coverage Δ
frontera/contrib/scrapy/schedulers/frontier.py	96.69% <100%> (ø)	✅
frontera/_version.py	30.14% <ø> (-1.48%)	❌
frontera/contrib/messagebus/kafka/async.py	15.28% <ø> (ø)	✅
frontera/utils/misc.py	68.18% <ø> (ø)	✅

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[sibiryakov]

that line is wrong, otherwise LGTM.

[sibiryakov]

Could you fix the tests @rolando ?

[sibiryakov]

Closing in favor #276