Make use of default key value when creating `Rack::Session::Encryptor`

[dentarg]

This default value

rack-session/lib/rack/session/constants.rb

Line 9 in 7a4e2dd

RACK_SESSION = 'rack.session'

rack-session/lib/rack/session/abstract/id.rb

Line 241 in 7a4e2dd

key: RACK_SESSION,

Isn't used when configuring the Encryptor class:

rack-session/lib/rack/session/cookie.rb

Lines 163 to 165 in 7a4e2dd

	encryptor_opts = {
	purpose: options[:key], serialize_json: options[:serialize_json]
	}

use Rack::Session::Cookie and Rack::Session::Cookie, key: "rack.session" is the same, both give you a session cookie with key rack.session, but their signature will be different:

rack-session/lib/rack/session/encryptor.rb

Lines 183 to 185 in 7a4e2dd

	def compute_signature(data)
	signing_data = data
	signing_data += @options[:purpose] if @options[:purpose]

[ioquatix]

Do you mind submitting a PR to resolve this issue?