Added tests for new jekyll doctor warning

test/source/_urls_differ_by_case_invalid/page1.html

@@ -0,0 +1,6 @@
+---
+title: About
+permalink: /about/
+---
+
+About the site

test/source/_urls_differ_by_case_invalid/page2.html

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+---
+title: About
+permalink: /About/
+---
+
+About the site

test/source/_urls_differ_by_case_valid/page1.html

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+---
+title: About
+permalink: /about/
+---
+
+About the site

test/test_doctor_command.rb

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+require 'helper'
+require 'jekyll/commands/doctor'
+
+class TestDoctorCommand < Test::Unit::TestCase
+  context 'urls only differ by case' do
+    setup do
+      clear_dest
+    end
+
+    should 'return success on a valid site/page' do
+      @site = Site.new(Jekyll.configuration({
+        "source" => File.join(source_dir, '/_urls_differ_by_case_valid'),
+        "destination" => dest_dir
+      }))
+      @site.process
+      output = capture_stderr do 
+         ret = Jekyll::Commands::Doctor.urls_only_differ_by_case(@site)
+         assert_equal false, ret
+      end
+      assert_equal "", output
+    end
+
+    should 'return warning for pages only differing by case' do
+      @site = Site.new(Jekyll.configuration({
+        "source" => File.join(source_dir, '/_urls_differ_by_case_invalid'),
+        "destination" => dest_dir
+      }))
+      @site.process
+      output = capture_stderr do 
+         ret = Jekyll::Commands::Doctor.urls_only_differ_by_case(@site)
+         assert_equal true, ret
+      end
+      assert_equal "\e[33m           Warning: The following URLs only differ by case. On a case-insensitive file system one of the URLs will be overwritten by the other: #{dest_dir}/about/index.html, #{dest_dir}/About/index.html\e[0m\n", output
+    end
+  end
+end