Fix "warning: URI.regexp is obsolete"

[y-yagi]

URI.regexp is obsolete now.

$ ruby -w -e 'require "net/http"; URI.regexp'
-e:1: warning: URI.regexp is obsolete

This changed to use URI::DEFAULT_PARSER.make_regexp that use inside URI.regexp instead of URI.regexp. https://github.com/ruby/uri/blob/6bc8f666219e519c1001673c15adab5182ff05ef/lib/uri/common.rb#L294-L297