Basic (multiple values per field) form content as dictionary.
form = FormContentDict()
form[key] -> [value, value, ...]
form.has_key(key) -> Boolean
form.keys() -> [key, key, ...]
form.values() -> [[val, val, ...], [val, val, ...], ...]
form.items() -> [(key, [val, val, ...]), (key, [val, val, ...]), ...]
form.dict == {key: [val, val, ...], ...}
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