Change the way of handling the different possible Content-Type
g5k-api returns two differents Content-Type, for a single item or for a collection of items.
Commit f9ae4214 introduced a regression by always returning "simple" JSON instead of the g5k-api's specifics Content-Type.
Now, the correct Content-Type will be returned, or application/json if asked.
Fixes: f9ae4214
Edited by Samir Noir