using docker client 1.8.3 to run `exec` against a docker daemon built from master fails

Client: 1.8.3

Daemon built from: 37da495d4cf30bebb17397dd9e103e6d3e74fe08

Command: `docker exec -it name bash`

Error:

```
Content-Type specified (text/plain) must be 'application/json'
```
