Documentation clarity for requested_columns

[josh-chamberlain]

Context

This is either a bug or a documentation issue!

• When I try to use the param requested_columns: 'id,title' on GET /data-requests, I get a 500 error
  • commenting out this param fixes the issue
• It's not clear to me whether accessing nested properties like locations.display_name should work, but ideally it would!

Requirements

• make it clearer how to use the requested_columns param on GET /data-requests, and other endpoints where it is usable, if the syntax is more complicated than "comma-delimited" seems to imply
• if something is broken, fix it!
• if nested properties are not intended to be accessible this way, consider adding that ability

Tests

• should probably test to make sure this param is working