:white_check_mark: add tests for FormData header
:white_check_mark: add tests for FormData header

When request body is a FormData object, don't set the Content-Type header.

When request body is not a FormData object, default the request header to application/json (existing behavior).

oliverwoodings/marty,KeKs0r/marty-lib,bigardone/marty,bigardone/marty,thredup/marty-lib,goldensunliu/marty-lib,Driftt/marty,kwangkim/marty,CumpsD/marty-lib,thredup/marty,martyjs/marty-lib,martyjs/marty,gmccrackin/marty-lib,martyjs/marty,kwangkim/marty,bigardone/marty,thredup/marty,Driftt/marty,kwangkim/marty,oliverwoodings/marty