more detailed warning for toAttribute in Document::setAttribute

[dginev]

Follow-up to #2465 , adding a little more detail to the warning message.

I encountered the warning while diagnozing something independent, on the example:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$$\begin{aligned}A \end{aligned}$$
\end{document}

Which with this PR will now print to the .latexml.log file the following.

@brucemiller if you have any suggestions how to handle the array-reference attribute, I can add that here if you prefer.

Warning:unexpected:ARRAY While setting "columnwidths", don't know how to encode ARRAY(0x5728582ae2f8) as an attribute value.
	at test.tex; line 5 col 2 - line 5 col 34
	$VAR1 = [
	          bless( [
	                   491521
	                 ], 'LaTeXML::Common::Dimension' )
	        ];
	In Core::Document[@0x572857366e40] /tmp/test.tex; from line 5 col 2 to line 5 col 34
	 <= Alignment[] <= Core::Document[@0x572857366e40] <= Core::Definition::Constructor[\lx@beg... <= ...
Warning:unexpected:ARRAY While setting "rowheights", don't know how to encode ARRAY(0x5728582ae4f0) as an attribute value.
	at test.tex; line 5 col 2 - line 5 col 34
	$VAR1 = [
	          bless( [
	                   447828
	                 ], 'LaTeXML::Common::Dimension' )
	        ];
	In Core::Document[@0x572857366e40] /tmp/test.tex; from line 5 col 2 to line 5 col 34
	 <= Alignment[] <= Core::Document[@0x572857366e40] <= Core::Definition::Constructor[\lx@beg... <= ...
 0.02 sec)

[dginev]

P.S. The reason I was studying that example is to check on the cause behind #2439