Session "Content-Type" header overriden by RequestBlob.headers

[liancheng]

Hit this issue while playing with the create issue JIRA REST API:

val session = requests.Session(
  auth = user -> password,
  headers = Map("Content-Type" -> "application/json")
)

val fields: ujson.Obj = ???

val response = session.post(
  url = "...",
  data = ujson.Obj("fields" -> ujson.Obj.from(fields))
)

The above snippet always got a 415 Unsupported Media Type error. Setting the "Content-Type" header explicitly while posting the requests fixes this issue:

val response = session.post(
  url = "...",
  headers = Map("Content-Type" -> "application/json")
  data = ujson.Obj("fields" -> ujson.Obj.from(fields)).render()
)

Further debugging showed that the "Content-Type: text/plain" header generated by StringRequestBlob took a higher precedence than the one defined in session.

[liancheng]

Fixed by #26.