DNF Repo.dump() function cannot be used as a .repo file for dnf due to
it writing baseurl and gpgkey as a list instead of a string. Add a new
function to write this in the correct format, and limited to the fields
we use.
Add a test for the new function.
Fix /projects/source/info to return an error 400 if a nonexistant TOML
source is requested. If JSON is used the error is part of the standard
response.
Update test_server.py to check for the correct error code.
(cherry picked from commit afa89ea657)
