[ruby/net-http] Update lib/net/http/header.rb

https://github.com/ruby/net-http/commit/826e008cfe

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

lib/net/http/header.rb

@@ -491,7 +491,7 @@ module Net::HTTPHeader
   alias canonical_each each_capitalized
 
   def capitalize(name)
-    name.to_s.split('-').map {|s| s.capitalize }.join('-')
+    name.to_s.split('-'.freeze).map {|s| s.capitalize }.join('-'.freeze)
   end
   private :capitalize