Reset m.optsum.feval in prfit! to show model has been fit

[dmbates]

• The prfit! method did not reset m.optsum.feval so the model was not recognized as having been fit.
• We should discuss how far we want to go in modifying m.optsum within prfit!.

Did behavior change? Did you add need features? If so, please update NEWS.md

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.

Should we release your changes right away? If so, bump the version:

• I've bumped the version appropriately

[palday]

@dmbates woops I didn't see this and just opened #801

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.27%. Comparing base (3f81dc2) to head (ba8928a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #800   +/-   ##
=======================================
  Coverage   97.27%   97.27%           
=======================================
  Files          35       35           
  Lines        3409     3410    +1     
=======================================
+ Hits         3316     3317    +1     
  Misses         93       93           

Flag	Coverage Δ
current	96.91% <100.00%> (+<0.01%)	⬆️
minimum	97.27% <100.00%> (+<0.01%)	⬆️
nightly	96.84% <100.00%> (+<0.01%)	⬆️

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[dmbates]

No problem. We can close this PR as #801 is more thorough.