Tests for Storage, Request and User

[groenroos]

As Storage was split out in #269, this PR brings tests for the methods in the various libs.

Testing for the remaining Storage methods isn't 100% coverage, as some parts of Storage are outdated/nonfunctional.

As a side effect, the Storage constructor was also standardised as per #268.

Closes #130, closes #268.

[codecov]

Codecov Report

Merging #320 (6d88dfc) into master (8f52bd7) will increase coverage by 1.53%.
The diff coverage is 98.90%.

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
+ Coverage   96.67%   98.21%   +1.53%     
==========================================
  Files          36       36              
  Lines        4367     4415      +48     
==========================================
+ Hits         4222     4336     +114     
+ Misses        145       79      -66     

Impacted Files	Coverage Δ
lib/Storage.js	91.00% <97.72%> (+7.43%)	⬆️
core/loadModel.js	100.00% <100.00%> (ø)
hooks/sapling/model/retrieve.js	100.00% <100.00%> (ø)
hooks/sapling/user/login.js	100.00% <100.00%> (ø)
lib/Request.js	98.92% <100.00%> (+11.65%)	⬆️
lib/User.js	100.00% <100.00%> (+3.33%)	⬆️

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