bug#39689: 26.3; browse-url-mail not supporting RFC6068 (UTF-8-Based Percent-Encoding)

[Vegard Vesterheim via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Emacs does not seem to correctly handle UTF-8-Based Percent-Encoding as
illustrated in Chapter 6.2 from RFC6068.

The command 
   emacs -Q -l browse-url -eval '(browse-url-mail "mailto:user@example.org?subject=caf%C3%A9&body=caf%C3%A9")'

should result in a message buffer with the string "café" insterted into the
body part of the message. Instead the string "café" is inserted.

I am running Ubuntu 18.04.3 LTS.

M-x emacs-version returns:
  "GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.30) of 2019-09-16" 

$ locale -a | grep -i utf
C.UTF-8
en_AG.utf8
en_AU.utf8
en_BW.utf8
en_CA.utf8
en_DK.utf8
en_GB.utf8
en_HK.utf8
en_IE.utf8
en_IL.utf8
en_IN.utf8
en_NG.utf8
en_NZ.utf8
en_PH.utf8
en_SG.utf8
en_US.utf8
en_ZA.utf8
en_ZM.utf8
en_ZW.utf8
nb_NO.utf8

$ env | grep LC
LC_MEASUREMENT=en_US.UTF-8
LC_PAPER=en_US.UTF-8
LC_MONETARY=en_US.UTF-8
LC_NAME=en_US.UTF-8
LC_ADDRESS=en_US.UTF-8
LC_NUMERIC=en_US.UTF-8
LC_TELEPHONE=en_US.UTF-8
LC_IDENTIFICATION=en_US.UTF-8
LC_TIME=nb_NO.utf8

$ env | grep LANG
LANG=en_US.UTF-8
GDM_LANG=en
NLS_LANG=NORWEGIAN_NORWAY.WE8ISO8859P1
LANGUAGE=en


--

Vennlig hilsen/Best regards
Vegard Vesterheim
Senior Software engineer
+47 48 11 98 98
vegard.vesterheim@uninett.no