Corrected valign: :bottom|:center spacing bug (Issue prawnpdf#169)

lib/prawn/text/formatted/box.rb

@@ -11,7 +11,7 @@ module Prawn
   module Text
     module Formatted
       # @group Stable API
-      
+
       # Draws the requested formatted text into a box. When the text overflows
       # the rectangle shrink to fit or truncate the text. Text boxes are
       # independent of the document y position.
@@ -293,7 +293,7 @@ def draw_fragment(fragment, accumulated_width=0, line_width=0, word_spacing=0) #
         end
 
         # @group Extension API
-        
+
         # Example (see Prawn::Text::Core::Formatted::Wrap for what is required
         # of the wrap method if you want to override the default wrapping
         # algorithm):
@@ -466,17 +466,20 @@ def process_vertical_alignment(text)
           # we need to wait until render() is called so that the fonts are set
           # up properly for wrapping. So guard with a boolean to ensure this is
           # only run once.
-          return if defined?(@vertical_alignment_processed) && @vertical_alignment_processed
+          return if @vertical_alignment_processed
           @vertical_alignment_processed = true
 
           return if @vertical_align == :top
           wrap(text)
 
+          line_padding = @line_height - (@ascender + @descender)
+          h = height - line_padding
+
           case @vertical_align
           when :center
-            @at[1] = @at[1] - (@height - height) * 0.5
+            @at[1] = @at[1] - (@height - h) * 0.5
           when :bottom
-            @at[1] = @at[1] - (@height - height) + @descender
+            @at[1] = @at[1] - (@height - h)
           end
           @height = height
         end