Disappearing whitespace in Para content, fixed.

Disappearing whitespace in Para content, fixed.
Problem:: For a Para, in output of content of two child elements
separated by whitespace, the space gets eaten. Also, for adjacent
text nodes separated by whitespace plus a comment. Cause:: I
didn't realize that running normalize-space() on a node that only
contains space returns an empty string :) Also, did not have the
code checking for comment or PI nodes correctly. Fix:: Added code
to correctly catch whitespacespace-only nodes, comments, and PIs.

Closes #1448860 and #1448856.