feat: Support RelayState binding by default during SSO

Per [OASIS SAML 2.0 standard](https://docs.oasis-open.org/security/saml/v2.0/saml-bindings-2.0-os.pdf):

> Some bindings define a "RelayState" mechanism for preserving and conveying state information. When
> such a mechanism is used in conveying a request message as the initial step of a SAML protocol, it
> places requirements on the selection and use of the binding subsequently used to convey the response.
> Namely, if a SAML request message is accompanied by RelayState data, then the SAML responder
> MUST return its SAML protocol response using a binding that also supports a RelayState mechanism, and
> it MUST place the exact RelayState data it received with the request into the corresponding RelayState
> parameter in the response.

In order to make standards-compliant usage of `RelayState` easier for implementing developers, this PR makes two changes:

1. It adds a default `RelayState` param mapping to the gem's `:idp_sso_service_url_runtime_params` config.
2. It enables the use of `RelayState` when `OmniAuth.config.test_mode` is enabled. - It does this by extending `OmniAuth::Strategy#mock_request_call` to add any POST `RelayState` params to the query string that will be used in the callback URL.

Tests have been added for both of these new behaviors.

lib/omniauth/strategies/saml.rb

@@ -13,7 +13,7 @@ def self.inherited(subclass)
       RUBYSAML_RESPONSE_OPTIONS = OneLogin::RubySaml::Response::AVAILABLE_OPTIONS
 
       option :name_identifier_format, nil
-      option :idp_sso_service_url_runtime_params, {}
+      option :idp_sso_service_url_runtime_params, { RelayState: 'RelayState' }
       option :request_attributes, [
         { :name => 'email', :name_format => 'urn:oasis:names:tc:SAML:2.0:attrname-format:basic', :friendly_name => 'Email address' },
         { :name => 'name', :name_format => 'urn:oasis:names:tc:SAML:2.0:attrname-format:basic', :friendly_name => 'Full name' },
@@ -118,6 +118,22 @@ def find_attribute_by(keys)
         nil
       end
 
+      def mock_request_call
+        # Per SAML 2.0, if a RelayState param is passed, IDPs "MUST place the exact RelayState
+        # data it received with the request into the corresponding RelayState parameter in the response."
+        #
+        # By default, the "mock" `OmniAuth::Strategy` implementation will forward along any URL params,
+        # so we can in turn take any POSTed RelayState params and put them in the GET query string:
+        query_hash = request.GET.merge!(additional_params_for_authn_request.slice('RelayState'))
+        query_string = Rack::Utils.build_query(query_hash)
+
+        request.set_header(Rack::QUERY_STRING, query_string)
+        request.set_header(Rack::RACK_REQUEST_QUERY_STRING, query_string)
+        request.set_header(Rack::RACK_REQUEST_QUERY_HASH, query_hash)
+
+        super
+      end
+
       private
 
       def request_path_pattern

spec/omniauth/strategies/saml_spec.rb

@@ -63,6 +63,33 @@ def post_xml(xml = :example_response, opts = {})
       end
     end
 
+    context 'with RelayState param' do
+      before do
+        post '/auth/saml', 'RelayState' => 'RELAY_STATE_VALUE'
+      end
+
+      it 'should get authentication page' do
+        expect(last_response).to be_redirect
+        expect(last_response.location).to match(
+          /\Ahttps:\/\/idp.sso.example.com\/signon\/29490\?SAMLRequest=.*&RelayState=RELAY_STATE_VALUE\z/,
+        )
+      end
+
+      context 'when test_mode is enabled' do
+        around do |example|
+          OmniAuth.config.test_mode = true
+          example.run
+        ensure
+          OmniAuth.config.test_mode = false
+        end
+
+        it 'should redirect to local saml callback page' do
+          expect(last_response).to be_redirect
+          expect(last_response.location).to eq('http://example.org/auth/saml/callback?RelayState=RELAY_STATE_VALUE')
+        end
+      end
+    end
+
     context "when the assertion_consumer_service_url is the default" do
       before :each do
         saml_options[:compress_request] = false