Fix bug when callback params is empty

This patch fixes an AttributeError exception in router.py.

Ubersmith send this callback payload for order modules:

    {'url': None, 'params': []}

Ubersmith serialize an empty array to a list instead of a dict.

Author: Frédéric Guillot

tests/test_router.py

@@ -72,3 +72,7 @@ def test_list_implemented_methods(self):
     def test_accept_callback_as_kwarg(self):
         self.router.invoke_method(module=self.module1, method='hello', params=[], env={},
                                   callback={'url': 'http://example.net', 'params': {'k1': 'v1', 'k2': 'v2'}})
+
+    def test_callback_has_empty_params_list(self):
+        self.router.invoke_method(module=self.module1, method='hello', params=[], env={},
+                                  callback={'url': 'http://example.net', 'params': []})

ubersmith_remote_module_server/router.py

@@ -37,6 +37,8 @@ def invoke_method(self, module, method, params=None, env=None, callback=None):
     def _build_request_context(self, callback):
         callback = callback or {}
         params = callback.get('params', {})
+        if isinstance(params, list):
+            params = dict()
         return RequestContext(callback_url=callback.get('url'),
                               module_id=params.get('module_id'),
                               device_id=params.get('device_id'),