Add RackHelpers.set_relying_party_in_request_env

* In order to streamline standard setups, we can provide the
	`set_relying_party_in_request_env` helper method; which can be
	included via `Warden::WebAuthn::RackHelpers` and used as needed

lib/warden/webauthn/rack_helpers.rb

@@ -8,6 +8,10 @@ module RackHelpers
       def relying_party_key
         "warden.webauthn.relying_party"
       end
+
+      def set_relying_party_in_request_env
+        request.env[relying_party_key] = relying_party
+      end
     end
   end
 end

test/warden/test_rack_helpers.rb

@@ -5,14 +5,30 @@
 class Warden::TestRackHelpers < Minitest::Test
   class TestClass
     include Warden::WebAuthn::RackHelpers
+
+    attr_accessor :request
+
+    def initialize
+      self.request = Rack::Request.new({})
+    end
   end
 
   class CustomizedClass
     include Warden::WebAuthn::RackHelpers
 
+    attr_accessor :request
+
+    def initialize
+      self.request = Rack::Request.new({})
+    end
+
     def relying_party_key
       "custom_relying_party"
     end
+
+    def relying_party
+      "dummy_relying_party_value"
+    end
   end
 
   def test_default_keys
@@ -22,4 +38,16 @@ def test_default_keys
   def test_custom_keys
     assert_equal "custom_relying_party", CustomizedClass.new.relying_party_key
   end
+
+  def test_raises_name_error_if_no_relying_party_method
+    assert_raises NameError do
+      TestClass.new.set_relying_party_in_request_env
+    end
+  end
+
+  def test_raises_uses_defined_relying_party_method
+    instance = CustomizedClass.new
+    instance.set_relying_party_in_request_env
+    assert_equal "dummy_relying_party_value", instance.request.env["custom_relying_party"]
+  end
 end