Fix sign in with multiple fields

[JuanitoFatas]

Hi Matt,

I was teaching a friend Rails and using Oath for authentication solution (of course). But I found the multiple fields auth does not work right now. There is already a solution in #55. I update his work so you could merge and release a new version of Oath.

can you just do a small write up about your solution. I assume the problem is that the params were being transformed twice. Was that the case?
-- halogenandtoast on #55 (comment)

Sign in with multiple fields:

# SessionsController
def create
  user = authenticate_session(session_params, email_or_username: [:email, :username])
end

In authenticate_session (L103) we already transformed the params

oath/lib/oath/controller_helpers.rb

Lines 101 to 107 in f850ea5

	def authenticate_session session_params, field_map = nil
	token_field = Oath.config.user_token_field
	params_hash = Oath.transform_params(session_params).symbolize_keys
	password = params_hash.fetch(token_field)
	user = Oath.lookup(params_hash.except(token_field), field_map)
	authenticate(user, password)
	end

and in Oath.lookup's self.config.find_method

oath/lib/oath.rb

Lines 86 to 91 in f850ea5

	def self.lookup(params, field_map)
	if params.present?
	fields = FieldMap.new(params, field_map).to_fields
	self.config.find_method.call(fields)
	end
	end

which calls Oath::Configuration#default_method that tries to transform again

oath/lib/oath/configuration.rb

Lines 55 to 60 in f850ea5

	def default_find_method
	->(params) do
	updated_params = Oath.transform_params(params)
	Oath.config.user_class.find_by(updated_params)
	end
	end

hence raises this error:

TypeError in SessionsController#create
wrong element type String at 0 (expected array)

The fix was not transformed twice.

cc @gracewashere Closes #55