Fix separate issues in JRuby's "native" "`dup` and `inspect` methods.

Update the stress test to exercise `proto3_optional` fields.
diff --git a/ruby/src/main/java/com/google/protobuf/jruby/RubyMessage.java b/ruby/src/main/java/com/google/protobuf/jruby/RubyMessage.java
index 25f9dca..a7699a0 100644
--- a/ruby/src/main/java/com/google/protobuf/jruby/RubyMessage.java
+++ b/ruby/src/main/java/com/google/protobuf/jruby/RubyMessage.java
@@ -202,7 +202,7 @@
     sb.append(cname).append(colon);
 
     for (FieldDescriptor fd : descriptor.getFields()) {
-      if (fd.hasPresence() && !fields.containsKey(fd)) {
+      if (fd.hasPresence() && !(fields.containsKey(fd) || builder.hasField(fd))) {
         continue;
       }
       if (addComma) {
@@ -550,8 +550,7 @@
       } else if (fields.containsKey(fieldDescriptor)) {
         dup.setFieldInternal(context, fieldDescriptor, fields.get(fieldDescriptor));
       } else if (this.builder.hasField(fieldDescriptor)) {
-        dup.fields.put(
-            fieldDescriptor,
+        dup.setFieldInternal(context, fieldDescriptor,
             wrapField(context, fieldDescriptor, this.builder.getField(fieldDescriptor)));
       }
     }
diff --git a/ruby/tests/stress.rb b/ruby/tests/stress.rb
index b688e8b..413d112 100755
--- a/ruby/tests/stress.rb
+++ b/ruby/tests/stress.rb
@@ -1,17 +1,18 @@
 #!/usr/bin/ruby
 
 require 'google/protobuf'
+require 'stress_pb'
 require 'test/unit'
 
 module StressTest
   pool = Google::Protobuf::DescriptorPool.new
   pool.build do
     add_message "TestMessage" do
-      optional :a,  :int32,        1
+      proto3_optional :a,  :int32,        1
       repeated :b,  :message,      2, "M"
     end
     add_message "M" do
-      optional :foo, :string, 1
+      proto3_optional :foo, :string, 1
     end
   end
 
@@ -29,9 +30,11 @@
       data = TestMessage.encode(m)
       100_000.times do
         mnew = TestMessage.decode(data)
-        mnew = mnew.dup
+        mnew2 = mnew.dup
         assert_equal m.inspect, mnew.inspect
         assert_equal data, TestMessage.encode(mnew)
+        assert_equal m.inspect, mnew2.inspect
+        assert_equal data, TestMessage.encode(mnew2)
       end
     end
   end