Speedup #generate_json by using case/when/end

* if/elsif comparing `obj.class` is significantly slower:
  ruby#668 (comment)
* The only case where an exact class check is needed so far is for String (ruby#667).
* Before: $ ruby -Ilib:ext benchmark/standalone.rb dump pure
JSON::Pure::Generator
truffleruby 24.2.0-dev-07b978e4, like ruby 3.2.4, Oracle GraalVM JVM [x86_64-linux]
Calculating -------------------------------------
      JSON.dump(obj)      6.426k (± 5.9%) i/s  (155.62 μs/i) -     32.395k in   5.064479s
      JSON.dump(obj)      6.380k (± 7.4%) i/s  (156.73 μs/i) -     31.806k in   5.021304s
      JSON.dump(obj)      6.276k (±10.5%) i/s  (159.33 μs/i) -     31.217k in   5.060762s
      JSON.dump(obj)      6.450k (± 7.0%) i/s  (155.05 μs/i) -     32.395k in   5.059538s
      JSON.dump(obj)      6.413k (± 6.2%) i/s  (155.93 μs/i) -     32.395k in   5.081573s
* After: $ ruby -Ilib:ext benchmark/standalone.rb dump pure
JSON::Pure::Generator
truffleruby 24.2.0-dev-07b978e4, like ruby 3.2.4, Oracle GraalVM JVM [x86_64-linux]
Calculating -------------------------------------
      JSON.dump(obj)      8.237k (± 5.0%) i/s  (121.41 μs/i) -     41.600k in   5.069507s
      JSON.dump(obj)      8.179k (± 5.1%) i/s  (122.26 μs/i) -     40.768k in   5.002035s
      JSON.dump(obj)      8.147k (± 7.9%) i/s  (122.74 μs/i) -     40.768k in   5.044840s
      JSON.dump(obj)      8.137k (± 6.9%) i/s  (122.90 μs/i) -     40.768k in   5.048690s
      JSON.dump(obj)      8.112k (±10.2%) i/s  (123.27 μs/i) -     39.936k in   5.023502s

lib/json/pure/generator.rb

@@ -307,20 +307,18 @@ def generate(obj)
 
         # Handles @allow_nan, @buffer_initial_length, other ivars must be the default value (see above)
         private def generate_json(obj, buf)
-          klass = obj.class
-          if klass == Hash
+          case obj
+          when Hash
             buf << '{'
             first = true
             obj.each_pair do |k,v|
               buf << ',' unless first
 
               key_str = k.to_s
-              if key_str.is_a?(::String)
-                if key_str.class == ::String
-                  fast_serialize_string(key_str, buf)
-                else
-                  generate_json(key_str, buf)
-                end
+              if key_str.class == String
+                fast_serialize_string(key_str, buf)
+              elsif key_str.is_a?(String)
+                generate_json(key_str, buf)
               else
                 raise TypeError, "#{k.class}#to_s returns an instance of #{key_str.class}, expected a String"
               end
@@ -330,7 +328,7 @@ def generate(obj)
               first = false
             end
             buf << '}'
-          elsif klass == Array
+          when Array
             buf << '['
             first = true
             obj.each do |e|
@@ -339,9 +337,13 @@ def generate(obj)
               first = false
             end
             buf << ']'
-          elsif klass == String
-            fast_serialize_string(obj, buf)
-          elsif klass == Integer
+          when String
+            if obj.class == String
+              fast_serialize_string(obj, buf)
+            else
+              buf << obj.to_json(self)
+            end
+          when Integer
             buf << obj.to_s
           else
             # Note: Float is handled this way since Float#to_s is slow anyway
@@ -432,8 +434,8 @@ def json_transform(state)
               result << state.indent * depth if indent
 
               key_str = key.to_s
-              key_json = if key_str.is_a?(::String)
-                key_str = key_str.to_json(state)
+              if key_str.is_a?(String)
+                key_json = key_str.to_json(state)
               else
                 raise TypeError, "#{key.class}#to_s returns an instance of #{key_str.class}, expected a String"
               end