JSON podspec raises warning when specifying flatten option for http source.

[alloy]

An example can be found at https://gist.github.com/alloy/f3e0ce3f72fd55cb90ee

~/t/C/mailcore2-ios-5 » pod spec lint mailcore2.podspec.json

 -> mailcore2 (0.4.5)
    - WARN  | [iOS] Incompatible `flatten` key(s) with `http` primary key for `source` attribute

Analyzed 1 podspec.

[!] The spec did not pass validation.

Thus it currently needs to be used with the --only-errors option to be able to pass.