bug#33429: 26.1.50; XML: Mistakenly complains about encoding

[Benny Riefenstahl]

When I open the attached file test.xml in "emacs -Q", Emacs complains in
*Warnings*:

  Warning (emacs): File contents detected as iso-latin-1.
    Consider adding an xml declaration with the encoding specified,
    or saving as utf-8, as mandated by the xml specification.

The file looks fine to me, I have even used nxml-insert-xml-declaration
to re-create the XML declaration.

Reproduced in a current build of emacs-26 and on master.

test.xml
Description: XML document

In GNU Emacs 26.1.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2018-09-05 built on riefenstahl-thinkpad
Repository revision: 9618e16efa5607664b794795fb80bdf5862fbe4b
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description:     Ubuntu 16.04.5 LTS