Don't mess with the original in String::substring

Before, substring would (temporarily) add a \0 in the original string at
the end of the substring, so the substring could be copied into a new
String object. However, now that the String::copy() method can work with
non-nul-terminated strings (by passing an explicit length), this trick
is not needed and we can just call the copy method instead.

Author: matthijskooijman

api/String.cpp

@@ -618,10 +618,7 @@ String String::substring(unsigned int left, unsigned int right) const
 	String out;
 	if (left >= len) return out;
 	if (right > len) right = len;
-	char temp = buffer[right];  // save the replaced character
-	buffer[right] = '\0';	
-	out = buffer + left;  // pointer arithmetic
-	buffer[right] = temp;  //restore character
+	out.copy(buffer + left, right - left);
 	return out;
 }