Whenever I call to_dict(flat=False) on a CombinedMultiDict, obtained from combining two MultiDicts with overlapping keys, only the values of the last MultiDict are returned.

# altered from documentation to make keys overlapping.
from werkzeug.datastructures import CombinedMultiDict, MultiDict
post = MultiDict([('foo', 'bar'),('foo', 'bar2')])
get = MultiDict([('foo', 'bar3')])
combined = CombinedMultiDict([get, post])
combined.getlist('foo')
>> ['bar3', 'bar', 'bar2']
combined.to_dict(flat=False)
>> {'foo': ['bar3']}

From a class that intends to combine two MultiDicts, I expect the result from one of its methods to be a combined result.
Environment:

• Python version: 3.6.8
• Werkzeug version: 1.0.1