libstdc++/5492: basic_string seems to fail to use template-defined allocator.

[Benjamin Kosnik]

Oh! I did some work on this and then never put it in the PR. It would be
great if you could take over this phil. I'll add in the notes later
today, but POST++ is allocating the string contents on the heap, so they
are not being saved in the persistent object.

I have a smaller example. I don't think this is a library bug anymore,
but a post++ bug. 

More details later