Usm 33 single user api

[marcotroisi]

Title: API for retrieving a Single User based on id

Note: this was already done and working in #34, so I used this ticket to do some small improvements and increase test coverage.

Ticket: #33

[codecov-io]

Codecov Report

Merging #37 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #37   +/-   ##
=======================================
  Coverage   77.27%   77.27%           
=======================================
  Files           3        3           
  Lines          66       66           
=======================================
  Hits           51       51           
  Misses         13       13           
  Partials        2        2

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