BUG #15342: pg_dump - XML with mixed content types generates invalidbackup file

The following bug has been logged on the website:

Bug reference:      15342
Logged by:          Ryan Lambert
Email address:      ryan@rustprooflabs.com
PostgreSQL version: 9.6.7
Operating system:   Ubuntu 16; Ubuntu 18; Raspbian (Pi)
Description:

Greetings!  

It seems that `pg_dump` is unable to provide a reliable database backups
that include specific combinations of XML data.  The following SQL Fiddle
creates a table with three rows of XML data.  The first row, "Document, no
DOCTYPE" is the only row of the three that will always load from a backup
from `pg_dump`.  I've tried this one a few sub-versions of 9.6 and 9.5.  

http://sqlfiddle.com/#!17/78a83/1/0

The second row added includes a DOCTYPE declaration in the XML.  Restoring
this row from pg_dump will fail unless you add `SET XML OPTION DOCUMENT;`.
Trying to restore the pg_dump file without adding `SET XML OPTION DOCUMENT`
returns:

```
ERROR:  invalid XML content
DETAIL:  line 2: StartTag: invalid element name
  <!DOCTYPE document SYSTEM "subjects.dtd">
   ^
CONTEXT:  COPY xml_doc, line 2, column data: "<?xml version="1.0"
standalone="no"?>
  <!DOCTYPE document SYSTEM "subjects.dtd">
  <document>
    <..."
```

The third row restores with the default setting but fails if `SET XML OPTION
DOCUMENT;` is set.

```
ERROR:  invalid XML document
DETAIL:  line 1: Start tag expected, '<' not found
abc<foo>bar</foo><bar>foo</bar>
^
CONTEXT:  COPY xml_doc, line 3, column data:
"abc<foo>bar</foo><bar>foo</bar>"
```

So it seems that if you have XML data that includes <!DOCTYPE> and other XML
that is just fragments... pg_dump won't work without manual tinkering and
headaches.

The specific data I use that is hanging me up is the QGIS layer style data
(stored in `public.layer_styles`).