bug#19551: 25.0.50; No lighter shown for `text-scale-mode' when it should be `+0'

[Drew Adams]

I don't know how or why this happens, but when buffer text is zoomed out
and back in, so that the resulting text scale is neutral (100%, zero
increase/decrease), the mode-line lighter should say `+0', as specified
in the code (`face-remap.el').  For some reason, the lighter just
disappears, as if the value of `text-scale-mode-lighter' were nil.

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'