READTABLE-CASE :invert does not invert uninterned symbols

[Mark Evenson]

​Ole Arendt reports:

This prevents loading a newer parenscript, which uses an
inverted readtable case via named-readtables:

Armed Bear Common Lisp 0.26.0-dev-svn-13273
Java 1.6.0_17 Sun Microsystems Inc.
Java HotSpot(TM) 64-Bit Server VM
Low-level initialization completed in 0.712 seconds.
Startup completed in 3.154 seconds.
Type ":help" for a list of available commands.

CL-USER(1): (setf (readtable-case *readtable*) :invert)
:invert
CL-USER(2): (string :cl)
"CL"
CL-USER(3): (string '#:cl)
"cl"

[Mark Evenson]

Fixed in r13508.

Opening #161 to track porting named-readtables.