Fix CrudApiAddProduct (#2114)

Author: sebastienros

src/BenchmarksApps/Mvc/benchmarks.crudapi.yml

@@ -27,7 +27,7 @@ scenarios:
       job: bombardier
       variables:
         path: /Products/1
-
+        
     # Add product
   ApiCrudAddProduct:
     application:
@@ -37,7 +37,8 @@ scenarios:
       variables:
         path: /Products
         verb: POST
-        bodyFile: https://raw.githubusercontent.com/aspnet/Benchmarks/main/src/BenchmarksApps/Mvc/crudproductbody.json
+    customHeaders: [ 'content-type: application/json' ]
+    bodyFile: https://raw.githubusercontent.com/aspnet/Benchmarks/main/src/BenchmarksApps/Mvc/crudproductbody.json
 
     # Update product
   ApiCrudUpdateProduct: