Change the way of handling the differents 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
3 jobs for bugs/12640 in 21 minutes and 33 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Lint
passed #926517
grid5000-docker
rubocop

00:01:34

 
  Test
passed #926518
grid5000-docker
test-for-buster

00:09:56

79.19%
 
  Deb
passed #926519
grid5000-docker
deb-for-buster

00:10:03