[Backport staging] Send Content-Type: application/json header for integration manager /register API (#9491)
[Backport staging] Send Content-Type: application/json header for integration manager /register API (#9491)

Co-authored-by: Michael Telatynski <94c9867e567d21b48f1c67ccab9a43d4b4ded84f@gmail.com>
matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk