Stop requiring stringio dynamically (#219)

`SourceFactory::create_from(String)` will always run the `require
'stringio'` operation. This prevents a multi-threaded JRuby application
from parsing xml on separate threads concurrently given that `require`
will pass through a synchronized piece of code.

An experiment in removing this `require` lead to a 10x performance
improvement on 10 threads parsing incoming strings on xml. For more
details see
logstash-plugins/logstash-filter-xml#83

lib/rexml/source.rb

@@ -1,6 +1,7 @@
 # coding: US-ASCII
 # frozen_string_literal: false
 
+require "stringio"
 require "strscan"
 
 require_relative 'encoding'
@@ -45,7 +46,6 @@ def SourceFactory::create_from(arg)
           arg.respond_to? :eof?
         IOSource.new(arg)
       elsif arg.respond_to? :to_str
-        require 'stringio'
         IOSource.new(StringIO.new(arg))
       elsif arg.kind_of? Source
         arg