Some of these issues became apparent when testing patch #1745 ( for issue #1720 )

  in some places will generate errors in PDF export, but not always.

Expected Behavior

[1] Should succeed in exporting PDF from Staff as well as Public . apps.
[2] Other named character entities should be encoded properly in EAD exports by being converted into numerical entities and only single lone ampersands should be escaped.
( i.e. the pattern in inner_xml function below needs to be more specific so it doesn't match entities. )

Current Behavior

Depending on where exactly entities are, this function will replace the ampersand with & entity:
https://github.com/archivesspace/archivesspace/blob/master/backend/app/converters/lib/xml_sax.rb#L221

Inserting several instances of   for testing, in one place this is output EAD fragment:
<physdesc id="aspace_b818d732f87bd233ec68c8d8d764afa3"><extent altrender="materialtype spaceoccupied">150 items</extent>&nbsp;<extent altrender="carrier">1 Hollinger box</extent>&nbsp;<dimensions>less than 1 linear foot</dimensions></physdesc>

Which causes an error when producing PDF for undefined nbsp entity.
However, elsewhere it is serialized as:

<physdesc> <dimensions id="aspace_d4005b5f554e4603d55436ef91de7fc4">less than &amp;nbsp; 1 &amp;nbsp; linear foot</dimensions> </physdesc>

And adding some arbitrary named character entities to title also produces escaped ampersands:
<unittitle>M&amp;atilde;ry &amp;Atilde;. Wilson p&amp;atilde;p&amp;eacute;rs</unittitle>

Note that all of those examples seem to display properly in PUI display ( they are known entities to HTML ) and after #1745 fix, they seem to work properly in PUI PDF download.
They also display properly in Staff view (HTML again) and only break when output in EAD/XML or PDF.

Possible Solution

1. More specific pattern match in inner_xml
2. translate other entities with HTMLEntities.new.decode as is done in Fix nbsp issue with PUI PDF generation by transforming nbsp to ASCII equivalent 160 #1745

But need to figure out why some ampersands are escaped and other are not first.

Steps to Reproduce (for bugs)

Context

Current behavior is inconsistent between Staff/PUI display and PDF/EAD serialization, and between Staff PDF and Public PDF exports.

Your Environment

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