Added custom headers support
Added custom headers support

Make it possible to send additional customs headers when submitting the
form. Most of the code was already there but it wasn’t used

omkarkhair/node-form-data,felixge/node-form-data,LinusU/node-form-data,sqreept/node-form-data,amberdixon/node-form-data,alexindigo/form-data,form-data/form-data