Fix exposing functions with int return types

[andrjohns]

Submission Checklist

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

Summary

This PR addresses #740 to fix exposing functions with int return types. I've also expanded the test to more thoroughly assess compatibility with different types

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):
Andrew Johnson

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

Merging #742 (df389a2) into master (463c2de) will increase coverage by 2.16%.
The diff coverage is 100.00%.

❗ Current head df389a2 differs from pull request most recent head 52e22f5. Consider uploading reports for the commit 52e22f5 to get more accurate results

@@            Coverage Diff             @@
##           master     #742      +/-   ##
==========================================
+ Coverage   86.09%   88.25%   +2.16%     
==========================================
  Files          12       12              
  Lines        4071     4071              
==========================================
+ Hits         3505     3593      +88     
+ Misses        566      478      -88     

Impacted Files	Coverage Δ
R/utils.R	74.01% <100.00%> (+1.96%)	⬆️

... and 4 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[rok-cesnovar]

Love it! Thanks @andrjohns.