Set list as an item in MultiDict

It would be nice if there can be a way to set a list as an item in a MultiDict object.

This is currently only possible if you re-create the object.

Examples

```
from chalice.app import MultiDict
m = MultiDict(None)
m["a"] = ["a1", "a2"]
print(m)
// MultiDict({'a': [['a1', 'a2']]}) # list is inserted in a list
```

```
from chalice.app import MultiDict
m = MultiDict({"a": ["a1", "a2"]})
print(m)
// MultiDict({'a': ['a1', 'a2']}) # list is OK, as it was provided during object creation
```

So maybe the correct functionality for `__setitem__` would be the following:
```
from chalice.app import MultiDict
m = MultiDict(None)
m["a"] = ["a1", "a2"]
print(m)
// MultiDict({'a': ['a1', 'a2']})
```

Start with the MultiDict class in chalice.app and inspect its current __setitem__ behavior alongside construction from an initial mapping. Reproduce the examples in the issue, then add or update a regression test covering assignment of a list and confirming the resulting values match construction-time behavior.

Written by the indexing model from the issue text.