Corrected after response for error handlers

Before this change after request functions were not correctly
invoked for error handlers.

tests/test_basic.py

@@ -768,6 +768,29 @@ def test_error_handling():
     assert b'forbidden' == rv.data
 
 
+def test_error_handling_processing():
+    app = flask.Flask(__name__)
+    app.config['LOGGER_HANDLER_POLICY'] = 'never'
+
+    @app.errorhandler(500)
+    def internal_server_error(e):
+        return 'internal server error', 500
+
+    @app.route('/')
+    def broken_func():
+        1 // 0
+
+    @app.after_request
+    def after_request(resp):
+        resp.mimetype = 'text/x-special'
+        return resp
+
+    with app.test_client() as c:
+        resp = c.get('/')
+        assert resp.mimetype == 'text/x-special'
+        assert resp.data == b'internal server error'
+
+
 def test_before_request_and_routing_errors():
     app = flask.Flask(__name__)