Buffer 64k in ZipTricks IO to improve performance (#21)

The zip_tricks gem uses less internal buffering than RubyZip, so wee
need to do some buffering of our own to get equal or better performance
than with the RubyZip IO for small rows.

lib/xlsxtream/io/zip_tricks.rb

@@ -3,25 +3,42 @@
 module Xlsxtream
   module IO
     class ZipTricks
+      BUFFER_SIZE = 64 * 1024
+
       def initialize(body)
         @streamer = ::ZipTricks::Streamer.new(body)
         @wf = nil
+        @buffer = ''
       end
 
       def <<(data)
-        @wf << data
+        @buffer << data
+        flush_buffer if @buffer.size >= BUFFER_SIZE
         self
       end
 
       def add_file(path)
-        @wf.close if @wf
+        flush_file
         @wf = @streamer.write_deflated_file(path)
       end
 
       def close
-        @wf.close if @wf
+        flush_file
         @streamer.close
       end
+
+      private
+
+      def flush_buffer
+        @wf << @buffer
+        @buffer.clear
+      end
+
+      def flush_file
+        return unless @wf
+        flush_buffer if @buffer.size > 0
+        @wf.close
+      end
     end
   end
 end