fix bad utf8 encodings by default (convert to u+FFFD)

this (temporarily, i hope) removes the fast path for handling
strings internally without escaping or replacing bad utf8

src/jsx_config.hrl

@@ -1,5 +1,5 @@
 -record(config, {
-    replaced_bad_utf8 = false,
+    strict_utf8 = false,
     escaped_forward_slashes = false,
     single_quoted_strings = false,
     unescaped_jsonp = false,