Ruby nested msg caps (#5564)

* Ruby nested msg caps

* nested types tests added

* test cases added for RubifyConstant

* extracted NestedMessage to TestNestedMessage

* removed nested tests with leading underscore

* extracted testLowercase to toplevel testLowercaseNested
diff --git a/ruby/tests/generated_code.proto b/ruby/tests/generated_code.proto
index 3b934bd..e811669 100644
--- a/ruby/tests/generated_code.proto
+++ b/ruby/tests/generated_code.proto
@@ -69,6 +69,11 @@
   C = 3;
 }
 
+message testLowercaseNested {
+  message lowercase{}
+}
+
+
 message TestUnknown {
   TestUnknown optional_unknown = 11;
   repeated TestUnknown repeated_unknown = 31;