Copying the result of an in-situ parsing into another value/document
currently requires that the original buffer - still holding the strings
from the parsing, outlives the  destination object as well.

In order to obtain a "full" copy of a GenericValue, this commit adds
an optional flag `copyConstStrings` to `CopyFrom`, which then forces
to take a copy of all embedded strings in the source value.

This solves the problem discussed in #962.