Fix handling of single-length inits for containers

[andrjohns]

Submission Checklist

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

Summary

Resolves the bug report in #855, which identified a more general issue that passing initial values for single-element containers would fail due to the auto_unbox setting in jsonlite treating them as scalars, resulting in cmdstan throwing a dimension-mismatch error.

This PR updates the processing of init-lists to check whether the initial value is for a single dimension-container and then format the value appropriately

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 #857 (881ef6e) into master (17678d5) will increase coverage by 0.00%.
Report is 31 commits behind head on master.
The diff coverage is 100.00%.

❗ Current head 881ef6e differs from pull request most recent head 50616d0. Consider uploading reports for the commit 50616d0 to get more accurate results

@@           Coverage Diff           @@
##           master     #857   +/-   ##
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  Coverage   88.19%   88.20%           
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  Files          12       12           
  Lines        4218     4221    +3     
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+ Hits         3720     3723    +3     
  Misses        498      498           

Files	Coverage Δ
R/args.R	97.77% <100.00%> (+0.01%)	⬆️

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[andrjohns]

@rok-cesnovar would you be able to have a look over this when you get a minute? It's a bugfix for #855

[rok-cesnovar]

Thanks, was a quick review :)

[andrjohns]

Brilliant, thanks!