Gattlib write char by uuid

[mweal-ed]

gattlib_write_char_by_uuid writes the incorrect buffer size. This changes gattlib_write_char_by_uuid to use buffer_len rather than sizeof(buffer) when writing so it writes the correct buffer size.

[oliviermartin]

May be this pull-request should be merged into #34?

[mweal-ed]

This is a separate unrelated commit. It is a one line change to bluez/gattlib_read_write.c:

@@ -222,7 +222,7 @@ int gattlib_write_char_by_uuid(gatt_connection_t* connection, uuid_t* uuid, cons
  		return ret;
  	}
  
 -	return gattlib_write_char_by_handle(connection, handle, buffer, sizeof(buffer));
 +	return gattlib_write_char_by_handle(connection, handle, buffer, buffer_len);
  }
  
  int gattlib_notification_start(gatt_connection_t* connection, const uuid_t* uuid) {

I am relatively new to this part of git and do not know how to separate it yet. I will look into it in the next few days.