Hash#merge doesn't work properly on a Dash

[marcelotto]

class Test < Hashie::Dash
    property :foo
end

h = Test.new(foo: 1)
h = h.merge(foo: 42)
h.foo # => 1, not 42 as expected for a Hash

The reason seems to be, that Hashie internally stores string keys, so Hash#merge only adds a new entry with the symbol key (which on the other hand is ignored by the Dash). The inclusion of StringifyKeys doesn't help.

DeepMerge doesn't work currently in this case, due to issue #99 and I wouldn't need it either. But even if it would work, this behaviour is a violation of Liskovs substitution principle.

[dblock]

Closed via #122.