Improve Template Support in JSON body

[fabricereix]

Currently, templates in json body is supported only for string

{
  "name": "{{name}}",
  "age": 30
}

the following file is not supported

{
  "name": "{{name}}",
  "age": {{age}}
}

Note that It is still possible to send this json body with a raw string.
But the content is not validated as json.

[fabricereix]

implemented in #117