Sending POST request resulting in "contentType.split is not a function"

I have no issues with using get requests, but once I add in a post request, it spits out warning saying

```
...node_modules/superagent/lib/node/index.js:981
if (contentType) contentType = contentType.split(';')[0];
^

TypeError: contentType.split is not a function
```

which I find kinda strange since tracing back to index.js, other strings with split function like the ones used in parsing cookies worked ok, dunno if its a bug or did I missed something.