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 39cdb8c

(cherry picked from commit d38528b)

test/middleware-mockserver.js

@@ -62,7 +62,7 @@ var mocks = {
  script: function( req, resp ) {
  if ( req.query.header === "ecma" ) {
  resp.writeHead( 200, { "content-type": "application/ecmascript" } );
- } else if ( req.query.header ) {
+ } else if ( "header" in req.query ) {
  resp.writeHead( 200, { "content-type": "text/javascript" } );
  } else {
  resp.writeHead( 200, { "content-type": "text/html" } );

test/unit/ajax.js

@@ -114,7 +114,7 @@ QUnit.module( "ajax", {
  return {
  create: function( options ) {
  options.crossDomain = true;
- return jQuery.ajax( url( "mock.php?action=script" ), options );
+ return jQuery.ajax( url( "mock.php?action=script&header" ), options );
  },
  success: function() {
  assert.ok( true, "success" );