fix rencode string length

Can not add magnet url to deluge through RPC interface
The rencode string length is wrong if magnet URL contents CJK utf8 string
Use byte length instead of string length

app/src/main/java/se/dimovski/rencode/RencodeOutputStream.java

@@ -212,7 +212,7 @@ public class RencodeOutputStream extends FilterOutputStream implements DataOutpu
      * Writes a {@link String}
      */
     public void writeString(String value) throws IOException {
-        int len = value.length();
+        int len = value.getBytes().length;
         if (len < TypeCode.EMBEDDED.STR_COUNT) {
             write(TypeCode.EMBEDDED.STR_START + len);
         } else {