code: function send(data) {
                  verifyState(this);
  
                  if (!/^(get|head)$/i.test(this.method)) {
                      var contentType = getHeader(this.requestHeaders, "Content-Type");
                      if (this.requestHeaders[contentType]) {
                          var value = this.requestHeaders[contentType].split(";");
                          this.requestHeaders[contentType] = value[0] + ";charset=utf-8";
                      } else if (!(data instanceof FormData)) {
                          this.requestHeaders["Content-Type"] = "text/plain;charset=utf-8";
                      }
  
                      this.requestBody = data;
                  }
  
                  this.errorFlag = false;
                  this.sendFlag = this.async;
                  this.readyStateChange(FakeXMLHttpRequest.OPENED);
  
                  if (typeof this.onSend == "function") {
                      this.onSend(this);
                  }
  
                  this.dispatchEvent(new sinon.Event("loadstart", false, false, this));
              }
docs: Currently treats ALL data as a DOMString (i.e. no Document)