Fix String conversion for unsigned char on Teensy 2.0

teensy/WString.cpp

 String::String(unsigned char c)
 {
 	init();
-	*this = (char)c;
+	char buf[4];
+	utoa(c, buf, 10);
+	*this = buf;
 }
 
 String::String(const int value, unsigned char base)