Serialize strings rather than returning literals during pretty-printing

[joewiz]

As reported by @Torcsi in eXist-db/exist#1420 and demonstrated by @dizzzz, given an element like <a b="&amp;#34;c"/>, eXide returns this as

3.1.x / eXide returns <a b="&#34;c"/>

whereas elsewhere this is returned with the ampersand properly escaped:

3.1.x / query via REST returns <a b="&amp;#34;c"/>
3.1.1 / doc retrieval via REST returns <a b="&amp;#34;c"/>

It appears that in eXide we return the literal string instead of serializing these strings. As a result, the browser is displaying & as &. Applying fn:serialize() to the strings should cause eXist to return "c which the browser will then display as the expected result, &34;c.