Switch to application json content type

[mtmail]

I had trouble running consumer.create against a Kong v0.14 instance. Server returned

# --- Caused by: ---
     # Kong::Error:
     #   {"fields":{"@entity":["at least one of these fields must be non-empty:
'custom_id', 'username'"]},"name":"schema violation","code":2,
"message":"schema violation (at least one of these fields must be
non-empty: 'custom_id', 'username')"}
     #   /ruby/path/.rvm/gems/ruby-2.5.1/gems/kong-0.3.2/lib/kong/client.rb:232:in 
`handle_error_response'

Of course one field (username) was filled but filling both made no difference. Used to work in Kong v0.11.

Some testing revealed switching the content-type for POST requests works. Tested against Kong v0.11 and v.0.14.

I can't quite explain which change in the Kong server logic caused this. Logic is likely in https://github.com/Kong/kong/commits/master/kong/api/arguments.lua Their upgrade/migration/changelog notes and issue tracker doesn't list anything relevant.

[nevalla]

Looking good! Thanks!