Cast hmset mapping values as strings

FakeRedis.hmset(key, mapping) doesn't currently convert the values in the mapping to strings:
```
In [1]: fr = fakeredis.FakeRedis()

In [2]: fr.hmset("foo", {"bar":1, "baz":True})
Out[2]: True

In [3]: fr.hgetall("foo")
Out[3]: {'bar': 1, 'baz': True}
```