set-frame-font with no KEEP-SIZE arg doesn't resize the frame, so (frame-width) is incorrect

[Drew Adams]

(set-frame-font <a font with a diff size>) -- no optional KEEP-SIZE arg
(nil).

The doc string says that this should cause the physical (pixel) size of the
frame to change, because the number of lines and columns is unchanged.

Bug: the physical frame size is _not_ changed with nil KEEP-SIZE.

After (set-frame-font <a font with a diff size>), the value of (frame-width)
is unchanged, which is correct per the doc string of set-frame-font. But
since the frame is still the same physical size, `frame-width' no longer
represents the width of the frame in characters -- it should always reflect
the frame font's character size: bigger characters for the same
`frame-width' should mean a wider frame; bigger characters for the same
`frame-pixel-width' (physical size) should mean a smaller `frame-width'
value.

Likewise, for height.

Note: In Emacs 20, (set-frame-font <a font with a diff size>) correctly
changes the physical frame size (since no KEEP-SIZE arg).  `frame-width'
always correctly represents the number of characters in the frame, as
measured by the size of the font that is the frame `font' property.