initialize parent param in default validator only if it's not present

[dm1try]

Maybe it's kind of tricky, but this one fixes #596

[dm1try]

I add CHANGELOG if this solution will be acceptable :)

[dblock]

Can you explain?

[dm1try]

There is only one case where parent element doesn't exist, see this spec and note.

params do
  optional :foo, type: Hash do
    optional :bar, default: 'foo-bar'
  end
end

We have optional param with default value in the group and no group params provided by client:

get '/group'

for example, in case

get '/group?foo'

we don't need initialize parent element

Anyway, changes in "default" validator was made by me for fixing #537 and before #545 PR( was added type for group/requires/optional params). Now it breaks default behavior for params with type Array. Why is tricky? Not sure, how should be implemented this behavior for params with type Array and whether it be responsibility of default validator :)

[dblock]

Merged.