Add support for .union() FHIRPath method. Enable tests for now().

[antvaset]

Two small changes for FHIRPath methods:

• Handling of .union().
• Enable the tests for the now() method (the tests pass with no changes required).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.68%. Comparing base (e00c396) to head (3e308e3). Report is 8 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1372      +/-   ##
============================================
+ Coverage     62.26%   63.68%   +1.41%     
+ Complexity     6951     2665    -4286     
============================================
  Files           492      492              
  Lines         27727    27736       +9     
  Branches       5506     5510       +4     
============================================
+ Hits          17265    17663     +398     
+ Misses         8198     7829     -369     
+ Partials       2264     2244      -20     

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