Using extended metadata requires that both source_url and issues_url are set

[jf647]

Supermarket (the app) will take the new extended metadata fields in any combo (source only, issues only, or both).

If you try to use stove to upload a cookbook that only has one of the extended metadata fields, it fails because it encodes the missing one as an empty hash in the JSON.

The fix seems simple - only populate the extended fields if the switch has been provided and the attribute is not empty.

PR to follow.

[directionless]

This issue is still present, and results in the confusing error message:

{"error_code"=>"INVALID_DATA", "error_messages"=>["The metadata is formatted incorrectly.", "Multipart POST part 'tarball' must contain a non-empty README."]}