Make absolute hrefs from urls

[gadomski]

Related Issue(s): stac-utils/pystac-client#113.

Description: The very basic case of make_absolute_href(href, None, None) was broken if href was a url. This fixes that breakage with a test case to demonstrate.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #697 (3092bdc) into main (9fa4b6c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #697   +/-   ##
=======================================
  Coverage   94.20%   94.20%           
=======================================
  Files          77       77           
  Lines       11120    11120           
  Branches     1337     1337           
=======================================
  Hits        10476    10476           
  Misses        466      466           
  Partials      178      178           

Impacted Files	Coverage Δ
tests/test_utils.py	100.00% <ø> (ø)
pystac/utils.py	95.38% <100.00%> (ø)

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[duckontheweb]

Thanks for catching and fixing this @gadomski !