[TEST] Roce ML post data test to use just JSON

This is temporary will we investigate why the test fails when SMILE is used

Original commit: elastic/x-pack-elasticsearch@55c7983a21

plugin/src/test/resources/rest-api-spec/test/ml/post_data.yaml

@@ -38,8 +38,13 @@ setup:
 
 ---
 "Test POST data job api, flush, close and verify DataCounts doc":
+  - skip:
+      features: ["headers"]
 
   - do:
+      #set the header so we won't randomize it
+      headers:
+        Content-Type: application/json
       xpack.ml.post_data:
         job_id: farequote
         body: