Flush JSON writer at end of records

[WardBrian]

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

Should close stan-dev/cmdstan#1349 (@jgabry @bgoodri I'd appreciate you trying it out!)

Intended Effect

Write to file at the end of JSON records (in particular, also at the end of the file, which is a record)

How to Verify

Side Effects

Documentation

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):

Simons Foundation

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/)

[jgabry]

Oops, I just submitted a PR for this too! I hadn't seen this one