Add tests to make sure. As we do not use standard library for formatting.

mrbgems/mruby-string-ext/test/string.rb

@@ -15,6 +15,7 @@ end
 
 assert('String#dump') do
   ("\1" * 100).dump     # should not raise an exception - regress #1210
+  "\0".inspect == "\"\\000\"" and
   "foo".dump == "\"foo\""
 end
 

test/t/string.rb

@@ -417,4 +417,5 @@ end
 
 assert('String#inspect') do
   ("\1" * 100).inspect  # should not raise an exception - regress #1210
+  "\0".inspect == "\"\\000\""
 end