Fix missing glyph width.

Prawn correctly used width of missing glyphs to layout text but didn't
encode the widths correctly in subset fonts. The resulted in characters
with missing glyphs to have 0 width. And even though Prawn drawn
Replacement Character glyphs they were stacking on top of each other
because their widths were wrong.

lib/prawn/fonts/ttf.rb

@@ -393,18 +393,18 @@ def embed_simple_font(reference, font, unicode_mapping)
           XHeight: x_height
         )
 
-        first_char = font.cmap.tables.first.code_map.index { |gid| !gid.zero? }
-        last_char = font.cmap.tables.first.code_map.rindex { |gid| !gid.zero? }
+        first_char, last_char = unicode_mapping.keys.minmax
         hmtx = font.horizontal_metrics
         widths =
-          font.cmap.tables.first.code_map[first_char..last_char].map do |gid|
-            if gid.zero?
+          (first_char..last_char).map do |code|
+            if unicode_mapping.key?(code)
+              gid = font.cmap.tables.first.code_map[code]
+              Integer(hmtx.widths[gid] * scale_factor)
+            else
               # These characters are not in the document so we don't ever use
               # these values but we need to encode them so let's use as little
               # sapce as possible.
               0
-            else
-              Integer(hmtx.widths[gid] * scale_factor)
             end
           end