Make punycode.toASCII not alter ASCII-safe input

This matches `ToASCII` as defined in https://tools.ietf.org/html/rfc3490#section-4.1:

> ToASCII never alters a sequence of code points that are all in the ASCII range to begin with (although it could fail). Applying the ToASCII operation multiple times has exactly the same effect as applying it just once.

Closes #59.

punycode.js

@@ -15,7 +15,7 @@ const delimiter = '-'; // '\x2D'
 
 /** Regular expressions */
 const regexPunycode = /^xn--/;
-const regexNonASCII = /[^\x20-\x7E]/; // unprintable ASCII chars + non-ASCII chars
+const regexNonASCII = /[^\0-\x7E]/; // non-ASCII chars
 const regexSeparators = /[\x2E\u3002\uFF0E\uFF61]/g; // RFC 3490 separators
 
 /** Error messages */

tests/tests.js

@@ -203,6 +203,11 @@ const testData = {
 			'decoded': '\uD83D\uDCA9.la',
 			'encoded': 'xn--ls8h.la'
 		},
+		{
+			'description': 'Non-printable ASCII',
+			'decoded': '\0\x01\x02foo.bar',
+			'encoded': '\0\x01\x02foo.bar'
+		},
 		{
 			'description': 'Email address',
 			'decoded': '\u0434\u0436\u0443\u043C\u043B\u0430@\u0434\u0436p\u0443\u043C\u043B\u0430\u0442\u0435\u0441\u0442.b\u0440\u0444a',