Corrected regression test for dynamic routing

no issue

- The reason for the test failure was:
- Bump in express version: 5a316c3c80)
- Which included bump of it's internal dependency `send` to version send@0.17.1: https://github.com/expressjs/express/releases/tag/4.17.0)
- Which had a change in how 'xml' type was interpreted: https://github.com/pillarjs/send/pull/154/files
- Because in Ghost documentation for dynamic routing, we clearly state to use `contetn_type: text/xml` instead of just `content_type:xml` the test was considered to be not up to date (ref. https://docs.ghost.org/api/handlebars-themes/routing/routes/)
- Returning `content-type: text/xml; charset=UTF-8` stays in line with the rest of Ghost codebase
- WARNING: when using `res.type('xml')` for whatever reason in the codebase be awere that express will send `application/xml` header which is not the convention used throughout the codebase

core/test/regression/site/site_spec.js

@@ -1632,7 +1632,7 @@ describe('Integration - Web - Site', function () {
                         '/about/': 'about',
                         '/podcast/rss/': {
                             templates: ['podcast/rss'],
-                            content_type: 'xml'
+                            content_type: 'text/xml'
                         },
                         '/cooking/': {
                             controller: 'channel',
@@ -3376,7 +3376,7 @@ describe('Integration - Web - Site', function () {
                         '/about/': 'about',
                         '/podcast/rss/': {
                             templates: ['podcast/rss'],
-                            content_type: 'xml'
+                            content_type: 'text/xml'
                         },
                         '/cooking/': {
                             controller: 'channel',