RapidXml

Rapid XML 1.12 interprets a zero, when specified as a string's length, as
indicating that the string is a C-string & its length should be calculated
appropriately.
This makes it difficult to add variable length data where it may have zero
length, but isn't necessarily a C-string. This may have also introduced
hidden bugs into users code. Zero is a valid length for data & should be
interpreted literally.

The attached patch defines a new constant `auto_size` to use instead of `0`
and modifies code appropriately. Note: The change was sufficient for my
usage. It probably requires some further work to update every occurrence.