Knockout hash keys instead of replacing with empty strings

[misdoro]

Currently there is no way to delete a hash key when merging two hashes.
Additionally, when using knock-out, when the child value is a hash, it will be replaced by an empty string, changing the data type of a value.

Update the code to knock-out hash keys instead of replacing them with an empty string.
I understand this is a breaking change as-is, happy to introduce an extra configuration flag for the new behavior.

Somewhat similar to #36 , happy that I was not the only one finding the current behavior inconsistent.

Usecase: for https://github.com/rubyconfig/config it is currently impossible to remove an optional hash from the common configuration.

This leads to impossibility of validating a schema like

optional(:server).schema do
    required(:hostname).value(:string)
    required(:port).value(:string)
end

where settings.yml looks like

server:
  host: localhost
  port: 1234

and overriding yml looks like

server: --