Always call update_function_configuration

Handle the case where a user completely removes the env vars
from their config.json.  We'll need this as more config options
for lambda are added.

chalice/awsclient.py

@@ -84,10 +84,16 @@ def update_function(self, function_name, zip_contents,
         lambda_client = self._client('lambda')
         return_value = lambda_client.update_function_code(
             FunctionName=function_name, ZipFile=zip_contents)
-        if environment_variables is not None:
-            lambda_client.update_function_configuration(
-                FunctionName=function_name,
-                Environment={"Variables": environment_variables})
+        if environment_variables is None:
+            environment_variables = {}
+        # We need to handle the case where the user removes
+        # all env vars from their config.json file.  We'll
+        # just call update_function_configuration every time.
+        # We're going to need this moving forward anyways,
+        # more config options besides env vars will be added.
+        lambda_client.update_function_configuration(
+            FunctionName=function_name,
+            Environment={"Variables": environment_variables})
         return return_value
 
     def get_role_arn_for_name(self, name):

tests/functional/test_awsclient.py

@@ -25,8 +25,11 @@ def test_deploy_rest_api(stubbed_session):
 
 
 def test_update_function_code_only(stubbed_session):
-    stubbed_session.stub('lambda').update_function_code(
+    lambda_client = stubbed_session.stub('lambda')
+    lambda_client.update_function_code(
         FunctionName='name', ZipFile=b'foo').returns({})
+    lambda_client.update_function_configuration(
+        FunctionName='name', Environment={'Variables': {}}).returns({})
     stubbed_session.activate_stubs()
     awsclient = TypedAWSClient(stubbed_session)
     awsclient.update_function('name', b'foo')