Correctly compare dates for PR retrieval; fix tests

[cjbarth]

The logic for comparing dates tries to compare a string to a Date object. This causes additional pages of PRs to never be retrieved. This has been fixed.

Also, constructing a date used the local-specific date instead of UTC, which means that the tests would only pass if one ran them from a UTC timezone. The code was corrected to build the dates using UTC.

I found this problem when working on node-saml/passport-saml#597

[codecov-commenter]

Codecov Report

Merging #301 (960dbb2) into master (d7ad51a) will decrease coverage by 0.19%.
The diff coverage is 10.00%.

❗ Current head 960dbb2 differs from pull request most recent head 0b306da. Consider uploading reports for the commit 0b306da to get more accurate results

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
- Coverage   39.08%   38.88%   -0.20%     
==========================================
  Files           8        8              
  Lines         394      396       +2     
==========================================
  Hits          154      154              
- Misses        240      242       +2     

Impacted Files	Coverage Δ
lib/src/Gren.js	4.72% <0.00%> (-0.05%)	⬇️
lib/src/_utils.js	95.71% <100.00%> (ø)

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[cjbarth]

@alexcanessa , would you be able to take a quick look at this PR? This issue affects node-saml and passport-saml, and I'd love to get this fix landed and released. If there is anything I can do to help the process along, please let me know.