JSON stringify for passed-in objects & a content-type param

Hi, i use atomic in a project and just stumbled across this. Passing objects will cause Object.prototype.toString to be called on the object somewhere in XMLHTTPRequest. It will then send [Object object], which is useless for my use case. Im new to GitHub, and i have no idea whether you might want to merge this into your repo, but here it is, just in case :)