Prevent adding 'content-type' header if one already exists

Fixes #19

- Added a `hasContentType` boolean to determine whether a `'Content-Type'` header already exists. (Should this be a helper function instead?)
- Renamed `args.headers['content-type']` to `args.headers['Content-Type']` because header names conventionally have the first letter of each word capitalized. [Sometimes lowercase can cause issues](https://github.com/cjohansen/Sinon.JS/issues/547).