[fix] Avoid requiring 'stringio' so early in boot process

On JRuby 9.4 this causes Rubygems initialisation which means realisation of init values. This screws with jruby-rack's own guarantees about env implied by its config values and can cause issues in boot hooks such as jruby/warbler#575 due to Rubygems being in a different state to what they expect.

Author: chadlwilson

CHANGELOG.md

@@ -4,6 +4,7 @@
 - update (bundled) rack to 2.2.20
 - Ensure rack boot process leaves ENV['GEM_PATH'] and Gem.paths in a consistent state
 - Remove undocumented and unsafe jruby.rack.env.gem_path = false option (unusable on Bundler 1.6+)
+- Fix unintended Rubygems initialization too early in boot process with JRuby 9.4+
 
 ## 1.2.5
 

src/main/ruby/jruby/rack/capture.rb

@@ -5,13 +5,11 @@
 # See the file LICENSE.txt for details.
 #++
 
-require 'stringio'
-
 module JRuby::Rack
   module Capture
     module Base
       def output
-        @output ||= begin; StringIO.new; end
+        @output ||= begin; require 'stringio' unless defined?(StringIO); StringIO.new; end
       end
 
       def capture
@@ -34,6 +32,7 @@ def store
     module Exception
       def output
         @output ||= begin
+          require 'stringio' unless defined?(StringIO)
           io = StringIO.new
           io.puts "An exception happened during JRuby-Rack startup", self.to_s
           io