Make web status text/plain

When I added the JSON targets in
I3410a4e4efd649732747b0502d66d212f50fa1bb I also changed the text
output mime-type to be application/text, which seemed to pair with
application/json.  But this means browsers prompt to download the file
rather than display it, which is not the intention.  Restore to
text/plain.

Change-Id: I6fa6597b2fcd05a4c33ba4932c966101daf98ecf

nodepool/tests/test_webapp.py

@@ -39,7 +39,7 @@ class TestWebApp(tests.DBTestCase):
             "http://localhost:%s/image-list" % port)
         f = urllib2.urlopen(req)
         self.assertEqual(f.info().getheader('Content-Type'),
-                         'application/text')
+                         'text/plain; charset=UTF-8')
         data = f.read()
         self.assertTrue('fake-image' in data)
 

nodepool/webapp.py

@@ -93,7 +93,7 @@ class WebApp(threading.Thread):
         if request.path.endswith('.json'):
             content_type = 'application/json'
         else:
-            content_type = 'application/text'
+            content_type = 'text/plain'
 
         response = webob.Response(body=output,
                                   content_type=content_type)