Fixed warnings

I use ruby-2.0.0 with -w option when running tests and I got many warnings that annoyed me a little. I fixed these warnings, here is the patch.

open4-1.3.0/lib/open4.rb:182: warning: shadowing outer local variable - a
open4-1.3.0/lib/open4.rb:213: warning: shadowing outer local variable - a
open4-1.3.0/lib/open4.rb:228: warning: shadowing outer local variable - key
open4-1.3.0/lib/open4.rb:225: warning: assigned but unused variable - ignored
open4-1.3.0/lib/open4.rb:258: warning: shadowing outer local variable - t
open4-1.3.0/lib/open4.rb:262: warning: shadowing outer local variable - t
open4-1.3.0/lib/open4.rb:268: warning: shadowing outer local variable - buf
open4-1.3.0/lib/open4.rb:340: warning: shadowing outer local variable - i
open4-1.3.0/lib/open4.rb:340: warning: shadowing outer local variable - stdin
open4-1.3.0/lib/open4.rb:345: warning: shadowing outer local variable - o
open4-1.3.0/lib/open4.rb:345: warning: shadowing outer local variable - stdout
open4-1.3.0/lib/open4.rb:349: warning: shadowing outer local variable - o
open4-1.3.0/lib/open4.rb:349: warning: shadowing outer local variable - stderr
open4-1.3.0/lib/open4.rb:384: warning: shadowing outer local variable - arg
open4-1.3.0/lib/open4.rb:384: warning: shadowing outer local variable - argv