When entries are added to a form's entry list through the "append an
entry" algorithm, their newlines are normalized, but entries can be
added to an entry list through other means. This change adds a final
newline normalization before serializing the form payload, since "append
an entry" cannot be changed because its results are observable through
the `FormData` object or through the `formdata` event.

This change additionally changes the input passed to the
`application/x-www-form-urlencoded` and `text/plain` serializers to be a
list of name-value pairs, where the values are strings rather than
`File` objects. This simplifies the serializer algorithms.

Closes whatwg#6247. Closes whatwg/url#562.

…a algorithm section.

See whatwg#6624 (review)