* lib/net/http.rb: new method Net::HTTPHeader#set_form_data and its alias #form_data=.
* lib/net/http.rb: Net::HTTPHeader#add_header -> add_field (adjustted to Ruby 1.8).
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@8362 b2dd03c8-39d4-4d8f-98ff-823fe69b080e