[ruby/net-http] Fix the regexp used to clean the host

Introduced in c1652035644

`/s` marks the regexp as encoded with Windows-31J which makes little
sense.

Nurse thinks the intent was to use `/m` for a multi-line regexp.

ruby/net-http@6c15342cdf

lib/net/http/generic_request.rb

@@ -143,7 +143,7 @@ def update_uri(addr, port, ssl) # :nodoc: internal use only
     end
 
     if host = self['host']
-      host.sub!(/:.*/s, ''.freeze)
+      host.sub!(/:.*/m, ''.freeze)
     elsif host = @uri.host
     else
      host = addr