Doesn't run ActiveSupport::JSON.decode(value) unless value starts with '{' and ends with '}'

This fixes my issue described here tombenner/nested-hstore/1

I did break one of the specs.  I'm not sure if this is desirable behavior or not...

  1) NestedHstore::Serializer with an array #deserialize deserializes
     Failure/Error: deserialized.should == @deserialized
       expected: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
            got: ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10"] (using ==)
     # ./spec/nested_hstore/serializer_spec.rb:13:in `it_deserializes'
     # ./spec/nested_hstore/serializer_spec.rb:69:in `block (4 levels) in <top (required)>'

So I patched the spec as well to expect strings instead of integers.