Lambda usage is broken

[mzahor]

I've created this issue as a general one for lambda issues.
Since we don't use lambda mode in our organization, it was never properly tested.
I think, the main 2 issues are lowercased keys and error handling.
Lowercased keys can be fixed by an additional callback. Error handling is a matter of documentation.
We may not have spare time to fix this in the nearest future, but we'll gladly accept a PR.

#15 #17 #16