Fix lambda stdout/stderr mocking (#851)

Originally, the code was setting sys.stdout and sys.stderr back to the
original, official forms, but this breaks idioms like mocking stdout to
capture printing output for tests. So instead, we will reset sys.stdout
and sys.stderr to what they were before running the lambda function, so
that in case someone is mocking stdout or stderr, their tests won't
break.

moto/awslambda/models.py

@@ -135,6 +135,8 @@ class LambdaFunction(object):
             print("Exception %s", ex)
 
         try:
+            original_stdout = sys.stdout
+            original_stderr = sys.stderr
             codeOut = StringIO()
             codeErr = StringIO()
             sys.stdout = codeOut
@@ -150,8 +152,8 @@ class LambdaFunction(object):
         finally:
             codeErr.close()
             codeOut.close()
-            sys.stdout = sys.__stdout__
-            sys.stderr = sys.__stderr__
+            sys.stdout = original_stdout
+            sys.stderr = original_stderr
         return self.convert(result)
 
     def invoke(self, request, headers):