Use/store redirect url fingerprint

[KrumBoychev]

Use initial redirect URL for fingerprint

[codecov-io]

Codecov Report

Merging #21 into master will decrease coverage by 1.97%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage    91.3%   89.33%   -1.98%     
==========================================
  Files           2        2              
  Lines          69       75       +6     
  Branches        9       10       +1     
==========================================
+ Hits           63       67       +4     
- Misses          3        4       +1     
- Partials        3        4       +1

Impacted Files	Coverage Δ
scrapy_deltafetch/middleware.py	89.04% <77.77%> (-2.01%)	⬇️

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[KrumBoychev]

The case can be handled with custom 'deltafetch_key' in meta, so this feature is not needed:

`import hashlib

request = scrapy.Request(url, callback=self.parse_item,
meta={'deltafetch_key': hashlib.sha1(original_url).hexdigest()})`