Better support for rvalue from-python conversions of shared_ptr:

always return a pointer that holds the owning python object *unless*
the python object contains a NULL shared_ptr holder of the right type.


[SVN r28947]

libs/python

@@ -1 +1 @@
-Subproject commit adb7b62a6207033933bd73fb8c9085130893b6ce
+Subproject commit 82563df3bff3fbe957a49af30ffa4c8eb4f36201