Adding multiple stylesheets with EXPORT_REVEAL_EXTRA_CSS property does not work as expected

[pingebat]

Declaring multiple extra scripts can be done with the
REVEAL_EXTRA_SCRIPT_BEFORE_SRC and
REVEAL_EXTRA_SCRIPT_SRC properties by handing over the
file names as a simple whitespace seperated "list" like in
the example below.

*** Presentation
    :PROPERTIES:
    :EXPORT_REVEAL_HLEVEL: 1
    :EXPORT_REVEAL_THEME: simple
    :EXPORT_REVEAL_REVEAL_JS_VERSION: 4
    :EXPORT_REVEAL_EXTRA_SCRIPT_BEFORE_SRC: scripts/script1.js scripts/script2.js
    :EXPORT_REVEAL_EXTRA_CSS: stylesheets/style2.css stylesheets/style1.css
    :END

However, adding multiple stylesheets
with the EXPORT_REVEAL_EXTRA_CSS property behaves
differently. Here the whitespace seperated list is interpreted as one string. And results in the output

<link rel="stylesheet" href="stylesheets/style1.css stylesheets/style2.css">

insteaf of:

<link rel="stylesheet" href="stylesheets/style1.css">
<link rel="stylesheet" href="stylesheets/style2.css">

By changing the splitting character "\n" on line 675 in
ox-reveal.el to " " I was able to produce the expected
behaviour.

I did not make a pull request because I am not sure if this
is the right way to fix it. There is also the possibilty that I
missed the right way to achieve what I am trying to do.
Either way, maybe someone more knowledgeable
can weigh in on this.