HTTP agent JSON Content-Type is not set by default when using custom headers

POST / HTTP/1.1
Host: localhost:8080
Accept: */*
DNT: 1
Content-Type: application/json
Content-Length: 14

{ "foo":"bar"}

POST / HTTP/1.1
Host: localhost:8080
Accept: */*
DNT: 1
Content-Length: 14
Content-Type: application/x-www-form-urlencoded

{ "foo":"bar"}