$lp_approx(), $mle() should return numeric vectors regardless of default draws format

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Ensure that $lp_approx() and $mle() methods return numeric vectors regardless of default draws format

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.24%. Comparing base (5576fd7) to head (897f6fe).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1190      +/-   ##
==========================================
+ Coverage   91.21%   91.24%   +0.03%     
==========================================
  Files          15       15              
  Lines        6125     6124       -1     
==========================================
+ Hits         5587     5588       +1     
+ Misses        538      536       -2     

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