Avoid transaction save when viewing a document

The strategy used by python's userdict copy method
(subclassed by PersistentDict) involves modifying the *original*
dict temporarily.
http://hg.python.org/cpython/file/2.7/Lib/UserDict.py#l27
This is not acceptable in the ZODB context:
we'd have a write-on-view, ConflictErrors like it's raining and Data.fs
growing out of control.