add Apache::Request#flush method for Kernel#spawn.

mod_ruby application cannot call Kernel#spawn.
Because Kernel#spawn requires $stdout.flush and mod_ruby replaces $stdout to Apache::Request instance (no flush method).

This pull-request fixes it.


An example:
```ruby
require "open3"

out, status = *Open3.capture2("ls")  #=> /home/yuya/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/open3.rb:211:in `spawn': undefined method `flush' for #<Apache::Request:0x007f5e76f5a628> (NoMethodError)
puts(out)
```