JP-3624: Fix a source of errors in resampled spec WCS

[mcara]

Resolves JP-3624

Closes #8485
Closes #8484

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• All comments are resolved
• Make sure the JIRA ticket is resolved properly

Regression test: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1479/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.98%. Comparing base (4179c09) to head (2a90f8d).
Report is 321 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #8511    +/-   ##
========================================
  Coverage   57.97%   57.98%            
========================================
  Files         387      388     +1     
  Lines       38830    38930   +100     
========================================
+ Hits        22513    22572    +59     
- Misses      16317    16358    +41     

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[emolter]

Single-line code change looks fine to me. Are the regression test differences for test_miri_lrs_slit_spec2 expected? There appear to be changes in a majority of the pixels, and some of them are several percent. Also, I assume you have already ensured that this change does indeed allow the exposures mentioned in the original ticket to be combined? If so, it might be helpful to add a note to the ticket to that effect.

[hbushouse]

Agree with @emolter that I'd very much like to see a visual before/after comparison of an example where it makes only a slight difference, as well as the example observation that was so far off as to trigger the ticket in the first place.

[hbushouse]

Examples of new results that are provided in the JP ticket look really good. I approve.