@phooper, Thanks a lot for making that screencast and explaining the issue in details.
I was able to duplicate the issue. It was only happening when I made the call using POSTMAN. If I use test_rest (a simple REST client that comes with df2 at http://your-instance.com/test_rest.html) then it worked fine. I was also getting a similar error when made a GET call using a payload (not an ideal situation but still breaks the script).
I have fixed these issues in the develop branch of df-core package. This will be available in our next release. For you, this may not be an issue if you make the call from your app vs POSTMAN. But if you want you can use the develop branch of df-core by making the following change in the composer.json file.
Replace: "dreamfactory/df-core": "~0.3.0",
with: "dreamfactory/df-core": "dev-develop as dev-master",
Then run composer update to pull the develop branch. Remember, this is develop branch and if you continue to use this you may run into some other issues.