Refactor

[jamieRowen]

this refactor was branched from my previous pull request #91 and constitutes some refactoring of the code base. this PR would either supersede it, or to be reviewed after #91 is merged

The main aim of the refactoring was to bring out bits of logic that were common across many places into simple functions with associated tests.

I also added a github action for running the tests.

refactored elemets:

• httr::GET logic
  • number of very similar calls to GET
  • created sm_get + 2 message helper functions for rate limit info
  • replaced instances in existing code base with calls to sm_get
  • tests for message helpers
  • can't test sm_get without an example survey or example responses from surveys
• httr request headers
  • request headers were formatted in the same way in each API call
  • abstracted out to standard_request_header() function
  • added associated tests
• factor out date formatting that is used on a number of request arguments throughout the package and added associated tests

[sfirke]

I don't have bandwidth to review but based on what you describe it sounds like a slam dunk of a PR. Note to Matt, I would merge #91 first and then this one will have a lot fewer relative changed lines to review.

[jamieRowen]

yeh the diff between #91 and #92 is a lot clearer