core: Don't always free the current id string in gdata_entry_get_id()
Currently, if you call gdata_entry_get_id() for the second time, the first returned id string is released even if the id hasn't been changed. This is unexpected and causes issues in several places of gvfsbackendgoogle.c, where g_strdup() has to be used as a workaround to prevent segfaults. Let's change the behavior to not free the current id, if it was not changed.
Closes: #31 (closed)