fix autodiff=AutoFiniteDiff() for TrustRegion

[oscardssmith]

Checklist

fixes #360

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7e0685a) 86.19% compared to head (cc3ebb7) 76.09%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #361       +/-   ##
===========================================
- Coverage   86.19%   76.09%   -10.11%     
===========================================
  Files          44       44               
  Lines        2608     2610        +2     
===========================================
- Hits         2248     1986      -262     
- Misses        360      624      +264     

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