Avoid mutating the input

[semenyukdmitry]

After the 1.4.2 update, passing a constant to jsonb_accessor started
mutating it, which might be really confusing to the user.

SCHEMA = {
  tags:  [:string, { array: true, default: [] }],
  title: [:string, { store_key: :t }]
}.freeze

class Widget < ActiveRecord::Base
  jsonb_accessor :data, SCHEMA
end

SCHEMA[:tags]   # => [:string] <- the options hash is gone
SCHEMA[:title]  # => [:string]

The PR fixes it by duplicating it before use. In addition to that, passing
a frozen a constant would now work fine.