Fix function download content-type not set correctly

Story: 2002979
Task: 22988

Change-Id: I998811a6bcf31f0a4e5125be6b9d50949199c0d8

qinling/api/controllers/v1/function.py

@@ -98,7 +98,7 @@ class FunctionsController(rest.RestController):
             raise exc.InputException('Failed to validate object in Swift.')
 
     @rest_utils.wrap_pecan_controller_exception
-    @pecan.expose()
+    @pecan.expose(content_type='application/zip')
     @pecan.expose('json')
     def get(self, id):
         """Get function information or download function package.
@@ -139,7 +139,6 @@ class FunctionsController(rest.RestController):
 
         pecan.response.app_iter = (f if isinstance(f, collections.Iterable)
                                    else FileIter(f))
-        pecan.response.headers['Content-Type'] = 'application/zip'
         pecan.response.headers['Content-Disposition'] = (
             'attachment; filename="%s"' % id
         )

qinling_tempest_plugin/tests/api/test_functions.py

@@ -48,6 +48,7 @@ class FunctionsTest(base.BaseQinlingTest):
         # Download function package
         resp, data = self.client.download_function(function_id)
         self.assertEqual(200, resp.status)
+        self.assertEqual('application/zip', resp['content-type'])
         self.assertEqual(os.path.getsize(self.python_zip_file), len(data))
 
         # Delete function