[plsql] xmlforest with optional AS

[szymanp23]

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PR Description:
In reference to comment.
It looks like there is a mistake in Oracle doc, because code compiles and works both with and without AS (on Oracle 12c).
For example
SELECT XMLELEMENT("Emp", XMLFOREST(e.dummy AS foo, e.dummy last_name)) "Emp Element" FROM dual e;
I modified parser and test case.

[pmd-test]

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[adangel]

btw - you should add your email address (you're using in the git commits) to github, because github right now still regonizes you as "first-time contributor"...