fix(stricttpl): move custom header interpolation after set content-type

[henripqt]

WHAT

Move the custom response header interpolation in the strict interface template after the Content-Type header is set in order to allow overriding it if needed

WHY

Currently one cannot define a custom Content-Type header if he wished.
For example if one wants to set a Content-Type of application/json; charset-utf8 for backwards compatibility support the only currently available option in openapi v3 would be to add a custom header of Content-Type with a default value like the following :

headers: 
  Content-Type:
    description: The unique ID of the request.
    schema:
      type: string
      default: application/json; charset=utf-8

However in in strict interface implementation custom headers are interpolated prior to the Content-Type being set and therefore setting a custom Content-Type becomes impossible.

HOW

By moving the custom headers interpolation after the call to set the Content-Type we allow greater flexibility.