class ActiveSupport::HashWithIndifferentAccess

def reverse_merge(other_hash)

hash.reverse_merge(a: 0, b: 1) # => {"a"=>nil, "b"=>1}
hash['a'] = nil
hash = ActiveSupport::HashWithIndifferentAccess.new

argument and returns a new hash with indifferent access as result:
Like +merge+ but the other way around: Merges the receiver into the 
def reverse_merge(other_hash)
  super(self.class.new(other_hash))
end