fixed two bugs in map.ex that did not correctly deal with nil or falsy values

[dylanradcliffe]

Found and resolved two suspected bugs in map.ex calculation of outstanding

1. Where actual has a missing key but the value is nil

iex(2)> outstanding(%{x: :a, y: nil}, %{x: :a, z: :b})
nil

2. Where actual has the key but wrong value and the expected value is falsy

iex(3)> outstanding(%{x: false}, %{x: 10})
nil

To me (2) definitively seems like a bug, but I could see (1) may not be depending on your expected semantics.

[matt-beanland]

Yes I agree that both of these are bugs and will merge the branch. Gold star awarded 🌟