set_multi wants 'STORED' even for values it refuses to store

[Bertrand Croq]

When set_multi decides that a value is too big to be stored, it still wants a 'STORED' from the server... the server is finally marked as dead.

Here is how to reproduce:
```
import memcache
c = memcache.Client(('localhost:11211', ))
assert not any(server.deaduntil for server in c.servers), 'servers are marked dead before set_multi!'
assert 'a' in c.set_multi({'a': 'a' * 1024 * 1025}), 'we were able to write more than 1Mb of data!'
assert not any(server.deaduntil for server in c.servers), 'servers are marked dead after set_multi!'
```
raises an ```AssertionError: servers are marked dead after set_multi!``` but it shouldn't