Refactor urlbox_client post method.

[AlanDonohoe]

What's this PR do?

Refactors urlbox_client post method.

Background context

This now uses the new _process_options method, as does all the other
get/head/delete methods.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@7fdfae1). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #35   +/-   ##
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  Coverage        ?   99.07%           
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  Files           ?        9           
  Lines           ?      432           
  Branches        ?        0           
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  Hits            ?      428           
  Misses          ?        4           
  Partials        ?        0           

Flag	Coverage Δ
unittests	99.07% <0.00%> (?)

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