Only save first PCDATA contents in the element
Only save first PCDATA contents in the element

This change fixes an important ordering issue - if element node has a PCDATA
child *after* other elements, it's impossible to tell which order the children
were in.

Since the goal of PCDATA embedding is to save memory when it's the only child,
only apply the optimization to the first child. This seems to fix all
roundtripping issues so the only caveat is that the DOM structure is different.

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml