Fix parsing the "Last-Modified" header in Blob Storage

[serhiy-storchaka]

It was parsed in local timezone.

Also simplify parsing the "Date" header in Storage.

[codecov]

Codecov Report

Merging #1874 (c888b85) into master (5fa0b5b) will decrease coverage by 0.03%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master    #1874      +/-   ##
==========================================
- Coverage   86.77%   86.73%   -0.04%     
==========================================
  Files          62       62              
  Lines        9444     9440       -4     
  Branches     1490     1486       -4     
==========================================
- Hits         8195     8188       -7     
- Misses        970      977       +7     
+ Partials      279      275       -4     

Flag	Coverage Δ
e2e	65.04% <72.72%> (-0.03%)	⬇️
unit	76.45% <72.72%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
neuromation/api/blob_storage.py	93.65% <66.66%> (-0.02%)	⬇️
neuromation/api/storage.py	92.30% <80.00%> (+0.33%)	⬆️
neuromation/cli/formatters/jobs.py	79.92% <0.00%> (-0.96%)	⬇️
neuromation/cli/ael.py	49.49% <0.00%> (ø)

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