* lib/net/http.rb (Net::HTTP#post, request_post, request): should set Content-Type: x-www-form-urlencoded by default.
* lib/net/http.rb (Net::HTTPHeader#content_type): should return nil when there's no Content-Type.
* lib/net/http.rb (Net::HTTPHeader#sub_type): should return nil when there's no sub Content-Type (e.g. "Content-Type: text").
* lib/net/http.rb (Net::HTTPHeader#type_params): wrongly failed when there's no Content-Type.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@10612 b2dd03c8-39d4-4d8f-98ff-823fe69b080e