Lower the severity of lower-case ruby enum to warning. Add conformance test for allow_alias with lower_case enums

ruby/ext/google/protobuf_c/message.c

@@ -697,6 +697,11 @@ VALUE build_module_from_enumdesc(EnumDescriptor* enumdesc) {
        upb_enum_next(&it)) {
     const char* name = upb_enum_iter_name(&it);
     int32_t value = upb_enum_iter_number(&it);
+    if (name[0] < 'A' || name[0] > 'Z') {
+      rb_warn("Enum value '%s' does not start with an uppercase letter "
+              "as is required for Ruby constants.",
+              name);
+    }
     rb_define_const(mod, name, INT2NUM(value));
   }