Adding optional headers parameter to Http client request

[blackgerman]

Context 🤔

App cast an http request to fetch version update file on a remote server. Issue is, when the file is protected (example: file is on a private remote repository), app cannot access the files without headers.

What did we do ? 🏮

Added optional header parameters to AppcastConfiguration object so that we can easily pass set of headers if we need to.

What to test ? 🧪

Test that the added parameter is really optional and doesn't break the basic flow.

[codecov[bot]]

Codecov Report

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

Comparison is base (212c232) 91.84% compared to head (8c97caa) 91.84%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #386   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files          10       10           
  Lines        1091     1092    +1     
=======================================
+ Hits         1002     1003    +1     
  Misses         89       89           

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[larryaasen]

@blackgerman Thanks for your PR and your suggestion about client headers, which I think is a useful idea. I decided to go a slightly different route with the implementation, so I will close this PR and make the updates myself.