added (failing) test for undesirable side-effects

use of `_read_tiddler_file` within `tiddler_put` modifies the incoming
tiddler, effectively doing a partial reset

confusingly, passing a new `Tiddler` instance to `_read_tiddler_file`
there leads to a StoreLockError - before we even get to that line! I
have no idea what's going on...