workaround FreeBSD/OSX IO bug for large uploads

workaround FreeBSD/OSX IO bug for large uploads

Under FreeBSD writing to the file in sync mode does not change current
position, so change position to the end of the file. Without this patch
multipart post requests with large data (image uploading) does not work
correctly:

  Status: 500 Internal Server Error
  bad content body
    /usr/local/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/utils.rb:347:in `parse_multipart'
    /usr/local/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/utils.rb:319:in `loop'
    /usr/local/lib/ruby/gems/1.8/gems/rack-1.0.0/lib/rack/utils.rb:319:in `parse_multipart'

File position behavior under FreeBSD  :

ruby -v
ruby 1.8.7 (2009-04-08 patchlevel 160) [i386-freebsd7]

irb(main):001:0> b  = File.new("abc", "w+")
=> #<File:abc>
irb(main):002:0>  b.sync = true
=> true
irb(main):004:0> b.write("abc")
=> 3
irb(main):005:0> b.pos
=> 0

Acked-by: Eric Wong <normalperson@yhbt.net>