fix search fields parameter typing and behavior

[philvarner]

Related Issue(s): #171

Description:

• fix fields typing
• fix fields behavior for GET and POST

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

Merging #184 (814fcc4) into main (2324110) will increase coverage by 1.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
+ Coverage   82.57%   83.87%   +1.29%     
==========================================
  Files           9        9              
  Lines         597      620      +23     
==========================================
+ Hits          493      520      +27     
+ Misses        104      100       -4     

Impacted Files	Coverage Δ
pystac_client/item_search.py	92.85% <100.00%> (+2.08%)	⬆️

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