Validate Content-Type Header for api/users

The content-type of Hub API requests used for user management, specifically for creating a user
is not validated and so the ‘text/plain’ type is accepted, where it must be ‘application/json’.
This commit adds validation for `Content-type` header for the /hub/api/users endpoint to only
allow requests with content-type as `application/json`

jupyterhub/tests/test_api.py

@@ -409,6 +409,10 @@ async def test_add_multi_user_bad(app):
     assert r.status_code == 400
     r = await api_request(app, 'users', method='post', data='[]')
     assert r.status_code == 400
+    r = await api_request(
+        app, 'users', method='post', data='{}', headers={"Content-Type": "text/plain"}
+    )
+    assert r.status_code == 403
 
 
 @mark.user