When I POST a file ending with a newline using EWW, the final newline
gets chomped off of the content.  This is because
mm-url-encode-multipart-form-data inserts CRLF unless it is at the
beginning of a line.  For file uploads, this behavior is incorrect.

To reproduce,

0. Upload a file ending in "\n" using EWW.

1. Observe the POST does not have CRLF between the file content and the
boundary.

Refer to https://www.rfc-editor.org/rfc/rfc2046#section-5.1.1 for
details.

I have not tested other kinds of html form posts.  I am not familiar
with what should be tested.  I did include tests for this specific bug
though.


In GNU Emacs 29.0.91 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.33, cairo version 1.16.0) of 2023-06-06 built on trent-reznor
Repository revision: b5eb43ba289519704c6cb0fe456038dcaec172c3
Repository branch: CRLF-before-noninitial-boundary
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Ubuntu 22.04.2 LTS

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