Switch content type of public keys back to text/plain

The public keys used to be served as text/plain. Since the cherrypy
refactor they are served as text/html. This can cause weird errors on
client side or trigger any content processing within the reverse
proxy. Switch that back to text/plain.

Change-Id: Ie30edc37f6e6136600d69e3b9eb74c86884a6879

tests/unit/test_web.py

@@ -263,6 +263,7 @@ class TestWeb(BaseTestWeb):
 
         resp = self.get_url("api/tenant/tenant-one/key/org/project.pub")
         self.assertEqual(resp.content, public_pem)
+        self.assertIn('text/plain', resp.headers.get('Content-Type'))
 
     def test_web_404_on_unknown_tenant(self):
         resp = self.get_url("api/tenant/non-tenant/status")

zuul/web/__init__.py

@@ -279,6 +279,7 @@ class ZuulWebAPI(object):
                                                   'project': project})
         resp = cherrypy.response
         resp.headers['Access-Control-Allow-Origin'] = '*'
+        resp.headers['Content-Type'] = 'text/plain'
         return job.data[0]
 
     @cherrypy.expose