Fix titleize_slug so already capitalized words are not dropped

Previously `titleize` used `capitalize!` which has the side effect of
returning `nil` for anything already starting with a capital letter. This
commit changes it to just `capitalize`.

Example, before:

A file "2016-01-01-This-is-a-title-with-Capitals.markdown" would return "Is A
Title With" for `post.title`

Example, after:

A file "2016-01-01-This-is-a-title-with-Capitals.markdown" will return "This Is A
Title With Capitals" for `post.title`

Tests added for `titleize_slug` in test_utils.rb

Fix problem introduced in 67f842546e

References #4525

lib/jekyll/utils.rb

@@ -20,7 +20,7 @@ module Jekyll
 
     def titleize_slug(slug)
       slug.split("-").map! do |val|
-        val.capitalize!
+        val.capitalize
       end.join(" ")
     end
 

test/test_utils.rb

@@ -206,6 +206,14 @@ class TestUtils < JekyllUnitTest
     end
   end
 
+  context "The \`Utils.titleize_slug\` method" do
+    should "capitalize all words and not drop any words" do
+      assert_equal "This Is A Long Title With Mixed Capitalization", Utils.titleize_slug("This-is-a-Long-title-with-Mixed-capitalization")
+      assert_equal "This Is A Title With Just The Initial Word Capitalized", Utils.titleize_slug("This-is-a-title-with-just-the-initial-word-capitalized")
+      assert_equal "This Is A Title With No Capitalization", Utils.titleize_slug("this-is-a-title-with-no-capitalization")
+    end
+  end
+
   context "The \`Utils.add_permalink_suffix\` method" do
     should "handle built-in permalink styles" do
       assert_equal "/:basename/", Utils.add_permalink_suffix("/:basename", :pretty)