Accepts strings as keys, only for declared attributes

Example:
  require 'lotus/validations'

  class Signin
    include Lotus::Validations

    attribute :email,    presence: true
    attribute :password, presence: true
  end

  signin = Signin.new('email' => 'user@example.org', 'password' => '123', 'unknown' => 'blah')
  signin.valid? # => true
  signin.to_h   # => { :email => 'user@example.org', :password => '123' }
Merge pull request #27 from lotus/attributes-with-key-strings-integration

Accepts strings as keys, only for declared attributes