Match httpDateString with ISO8601 formats

[kusakusakusa]

Checklist

• [ ] New Extension
• [ ] New Test
• [ ] Changed more than one extension, but all changes are related
• [x] Trivial change (doesn't require changelog)

Default timestamp from http json responses for Ruby on Rails follows ISO8601 format.

[EZSwiftExtensionsBot]

	2 Messages
:book:	Executed 193 tests, with 0 failures (0 unexpected) in 5.549 (5.752) seconds
:book:	Executed 178 tests, with 0 failures (0 unexpected) in 5.938 (6.122) seconds

Generated by :no_entry_sign: Danger

[codecov-io]

Codecov Report

Merging #426 into master will increase coverage by 0.93%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
+ Coverage   40.44%   41.38%   +0.93%     
==========================================
  Files          49       50       +1     
  Lines        2193     2216      +23     
==========================================
+ Hits          887      917      +30     
+ Misses       1306     1299       -7

Impacted Files	Coverage Δ
Sources/DateExtensions.swift	96.18% <100%> (+0.38%)	:arrow_up:
Sources/FileManagerExtension.swift	100% <0%> (ø)
Sources/UIViewExtensions.swift	7.27% <0%> (+3.74%)	:arrow_up:

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[Khalian]

Moreover, these changes also need changelog entries.