Fix jruby encoding in new thread

ruby/src/main/java/com/google/protobuf/jruby/Utils.java

@@ -36,7 +36,9 @@ import com.google.protobuf.ByteString;
 import com.google.protobuf.DescriptorProtos.FieldDescriptorProto;
 import com.google.protobuf.Descriptors.FieldDescriptor;
 import java.math.BigInteger;
+import org.jcodings.Encoding;
 import org.jcodings.specific.ASCIIEncoding;
+import org.jcodings.specific.UTF8Encoding;
 import org.jruby.*;
 import org.jruby.exceptions.RaiseException;
 import org.jruby.ext.bigdecimal.RubyBigDecimal;
@@ -141,12 +143,12 @@ public class Utils {
           throw createInvalidTypeError(context, "boolean", fieldName, value);
         break;
       case BYTES:
-        value = validateAndEncodeString(context, "bytes", fieldName, value, "Encoding::ASCII_8BIT");
+        value = validateAndEncodeString(context, "bytes", fieldName, value, ASCIIEncoding.INSTANCE);
         break;
       case STRING:
         value =
             validateAndEncodeString(
-                context, "string", fieldName, symToString(value), "Encoding::UTF_8");
+                context, "string", fieldName, symToString(value), UTF8Encoding.INSTANCE);
         break;
       case MESSAGE:
         if (value.getMetaClass() != typeClass) {
@@ -383,11 +385,11 @@ public class Utils {
       String fieldType,
       String fieldName,
       IRubyObject value,
-      String encoding) {
+      Encoding encoding) {
     if (!(value instanceof RubyString))
       throw createInvalidTypeError(context, fieldType, fieldName, value);
 
-    value = ((RubyString) value).encode(context, context.runtime.evalScriptlet(encoding));
+    value = ((RubyString) value).encode(context, context.runtime.getEncodingService().convertEncodingToRubyEncoding(encoding));
     value.setFrozen(true);
     return value;
   }