[Swift 5] fix Multipart FormData encoding and add support for Form URLEncoded enconding

[4brunu]

Previously it was only possible to use the encoding multipart/form-data when sending files.
With this PR that is fixed.

And it also adds support for 'application/x-www-form-urlencoded' fixing #8169

This PR also contains some fixes on code formatting.

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@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)

[4brunu]

CI is failing, but I think it's not related to this PR.

$ echo "$DOCKER_HUB_PASSWORD" | docker login --username=$DOCKER_HUB_USERNAME --password-stdin;
Must provide --username with --password-stdin
The command "echo "$DOCKER_HUB_PASSWORD" | docker login --username=$DOCKER_HUB_USERNAME --password-stdin;" failed and exited with 1 during .

Your build has been stopped.

[wing328]

Agreed it's not related.