Resolve state issue for url_for with forced scheme

This fixes #1596

tests/test_helpers.py

@@ -627,6 +627,16 @@ class TestLogging(object):
                                'index',
                                _scheme='https')
 
+    def test_url_for_with_alternating_schemes(self):
+        app = flask.Flask(__name__)
+        @app.route('/')
+        def index():
+            return '42'
+        with app.test_request_context():
+            assert flask.url_for('index', _external=True) == 'http://localhost/'
+            assert flask.url_for('index', _external=True, _scheme='https') == 'https://localhost/'
+            assert flask.url_for('index', _external=True) == 'http://localhost/'
+
     def test_url_with_method(self):
         from flask.views import MethodView
         app = flask.Flask(__name__)