curlhttpsink: Enable content type changes

Makes it possible to change content type without having to
reinitialize the element, typically after reset.

gstreamer/gst-plugins-bad!41
4 jobs for merge_contenttype_changes in 5 minutes and 17 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #57518
gst indent

00:00:29

passed #57517
manifest

00:00:22

 
  Build
passed #57519
build fedora x86_64

00:02:30

 
  Full Builds
passed #57520
build android arm64 api28

00:02:17