Fix "TypeError: unhashable type" when calling json_request

This bug was introduced as part of #43.  `dry_run` was appended to
construct_payload parameters but there was a single instance where
construct_payload was executed and relying on `is_json` parameter's
default value.
Merge pull request #46 from dkong/fix_json_request

Fix "TypeError: unhashable type" when calling json_request