Add more unit tests for connection prelude

This adds more code coverage to ensure both RESP2 and RESP3
cases have the right prelude.

Author: stanhu

test/redis_client/config_test.rb

@@ -64,19 +64,46 @@ def test_ipv6_uri
       assert_equal "::1", config.host
     end
 
-    def test_user_password_uri
+    def test_resp2_user_password_uri
+      config = Config.new(protocol: 2, url: "redis://username:[email protected]")
+      assert_equal "example.com", config.host
+      assert_equal 6379, config.port
+      assert_equal "username", config.username
+      assert_equal "password", config.password
+      assert_equal 0, config.db
+      refute_predicate config, :ssl?
+      assert_equal [%w[AUTH username password]], config.connection_prelude
+    end
+
+    def test_resp3_user_password_uri
       config = Config.new(url: "redis://username:[email protected]")
       assert_equal "example.com", config.host
       assert_equal 6379, config.port
       assert_equal "username", config.username
       assert_equal "password", config.password
       assert_equal 0, config.db
       refute_predicate config, :ssl?
+      assert_equal [%w[HELLO 3 AUTH username password]], config.connection_prelude
     end
 
-    def test_frozen_prelude
+    def test_resp2_frozen_prelude
+      config = Config.new(protocol: 2, url: "redis://username:[email protected]")
+      prelude = config.connection_prelude
+
+      assert_equal [%w[AUTH username password]], prelude
+      assert_equal true, prelude.frozen?
+      assert_equal true, (prelude.all? { |commands| commands.frozen? })
+
+      prelude.each do |commands|
+        assert_equal true, (commands.all? { |arg| arg.frozen? })
+      end
+    end
+
+    def test_resp3_frozen_prelude
       config = Config.new(url: "redis://username:[email protected]")
       prelude = config.connection_prelude
+
+      assert_equal [%w[HELLO 3 AUTH username password]], prelude
       assert_equal true, prelude.frozen?
       assert_equal true, (prelude.all? { |commands| commands.frozen? })
 
@@ -85,14 +112,26 @@ def test_frozen_prelude
       end
     end
 
-    def test_simple_password_uri
+    def test_resp2_simple_password_uri
+      config = Config.new(protocol: 2, url: "redis://[email protected]")
+      assert_equal "example.com", config.host
+      assert_equal 6379, config.port
+      assert_equal "default", config.username
+      assert_equal "password", config.password
+      assert_equal 0, config.db
+      refute_predicate config, :ssl?
+      assert_equal [%w[AUTH password]], config.connection_prelude
+    end
+
+    def test_resp3_simple_password_uri
       config = Config.new(url: "redis://[email protected]")
       assert_equal "example.com", config.host
       assert_equal 6379, config.port
       assert_equal "default", config.username
       assert_equal "password", config.password
       assert_equal 0, config.db
       refute_predicate config, :ssl?
+      assert_equal [%w[HELLO 3 AUTH default password]], config.connection_prelude
     end
 
     def test_simple_password_uri_empty_user
@@ -103,6 +142,7 @@ def test_simple_password_uri_empty_user
       assert_equal "password", config.password
       assert_equal 0, config.db
       refute_predicate config, :ssl?
+      assert_equal [%w[HELLO 3 AUTH default password]], config.connection_prelude
     end
 
     def test_percent_encoded_password_uri
@@ -114,6 +154,7 @@ def test_percent_encoded_password_uri
       assert_equal "p@ssw0rd", config.password
       assert_equal 12, config.db
       refute_predicate config, :ssl?
+      assert_equal [%w[HELLO 3 AUTH default p@ssw0rd], %w[SELECT 12]], config.connection_prelude
     end
 
     def test_rediss_url
@@ -124,13 +165,16 @@ def test_rediss_url
       assert_nil config.password
       assert_equal 0, config.db
       assert_predicate config, :ssl?
+      assert_equal [%w[HELLO 3]], config.connection_prelude
     end
 
     def test_trailing_slash_url
       config = Config.new(url: "redis://example.com/")
       assert_equal 0, config.db
+      assert_equal [%w[HELLO 3]], config.connection_prelude
       config = Config.new(url: "redis://[::1]/")
       assert_equal 0, config.db
+      assert_equal [%w[HELLO 3]], config.connection_prelude
     end
 
     def test_overriding
@@ -150,6 +194,7 @@ def test_overriding
       assert_equal "hunter2", config.password
       assert_equal 5, config.db
       assert_predicate config, :ssl?
+      assert_equal [%w[HELLO 3 AUTH george hunter2], %w[SELECT 5]], config.connection_prelude
     end
 
     def test_server_url