Pressing enter in the sandbox mode will now lead to submit the form.

From other tools like DHC or POSTMAN I was used to it, that pressing the enter key, the request was submitted. In the sandbox mode, when you clicked enter, there always was added a new header, as the button was the first appearing in the html markup and if a button has no type, the type is submit by default. Thus, I just added a type attribute to avoid that. 
Hope this is fine :)