Dash with default hash will use singleton shared amongst all instances

[msaffitz]

Here's a simple test case:

require 'hashie'
class Foobar < Hashie::Dash
  property :b, default: Hash.new
end

a = Foobar.new
a.b[:bar] = 'cat'

b = Foobar.new

I'd expect b to be {}, but actually is {b: { bar: 'cat'}}

Deferred defaults (issue #34) would help, but the default objects should probably be cloned by each new instance.