Tests: Fix tests for not auto-executing scripts without dataType

Tests: Fix tests for not auto-executing scripts without dataType

Two issues are fixed in testing for responses with a script Content-Type not
getting auto-executed unless an explicit `dataType: "script"` is provided:
* the test is now using a correct "text/javascript" Content-Type; it was using
  "text/html" until now which doesn't really check if the fix works
* the Node.js based version of the tests didn't account for an empty `header`
  query string parameter

Closes gh-4824
Ref gh-2432
Ref gh-2588
Ref 39cdb8c9aa0fde68f733553ba050a2ba9d86474c