FI-2567: Fix client instance variables

[Jammjammjamm]

Summary

FHIR/HTTP clients were incorrectly using call instead of send when delegating methods to the runnable in which they are defined.

Testing Guidance

The unit tests are adequate to demonstrate this failure. Check out 958ef0d28b431d2d01d7cb651d503182b2e8f37b to see the tests fail.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.69%. Comparing base (e7663b7) to head (985a7f5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #464      +/-   ##
==========================================
+ Coverage   77.68%   77.69%   +0.01%     
==========================================
  Files         233      233              
  Lines       11591    11591              
  Branches     1087     1087              
==========================================
+ Hits         9004     9006       +2     
+ Misses       1967     1965       -2     
  Partials      620      620              

Flag	Coverage Δ
backend	93.96% <100.00%> (+0.05%)	⬆️
frontend	70.77% <ø> (ø)

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