Bug 33290: Fix incorrect variable in http-client.js

Bug 33290: Fix incorrect variable in http-client.js

Silly error from
  commit 821808ec31176d662e518418c9ff8a71948e1a4b
  Bug 32939: Use APIClient to replace PATCH requests

+        const body = params.body
+            ? typeof str === "string"
+                ? params.body
+                : JSON.stringify(params.body)
+            : undefined;

  typeof str === "string"
must be
  typeof params.body === "string"

Test plan:
With this patch, bug 33289 is not working properly. The body contains
  "pref_var=value"
instead of
  pref_var=value
and the syspref `var` contains `value"`

Sponsored-by: BULAC - http://www.bulac.fr/
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 17de3a6feeed8237108c76ac2bf8e1cfddffc127)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>