Fixed problem identified by Georg. The special-case in-place code for replace

Fixed problem identified by Georg.  The special-case in-place code for replace
made a copy of the string using PyString_FromStringAndSize(s, n) and modify
the copied string in-place.  However, 1 (and 0) character strings are shared
from a cache.  This cause "A".replace("A", "a") to change the cached version
of "A" -- used by everyone.

Now may the copy with NULL as the string and do the memcpy manually.  I've
added regression tests to check if this happens in the future.  Perhaps
there should be a PyString_Copy for this case?