Fix rubocop errors & add one more test for slugify 'ascii' mode (#4680)

.rubocop.yml

@@ -23,7 +23,7 @@ Metrics/ClassLength:
     - !ruby/regexp /test\/.*.rb$/
   Max: 300
 Metrics/CyclomaticComplexity:
-  Max: 8
+  Max: 9
 Metrics/LineLength:
   Exclude:
     - !ruby/regexp /features\/.*.rb/

lib/jekyll/utils.rb

@@ -200,7 +200,7 @@ module Jekyll
           # "._~!$&'()+,;=@" is human readable (not URI-escaped) in URL
           # and is allowed in both extN and NTFS.
           SLUGIFY_PRETTY_REGEXP
-        when 'ascii'
+        when "ascii"
           # For web servers not being able to handle Unicode, the safe
           # method is to ditch anything else but latin letters and numeric
           # digits.

test/test_utils.rb

@@ -199,7 +199,10 @@ class TestUtils < JekyllUnitTest
     end
 
     should "replace everything else but ASCII characters" do
-      assert_equal "the-config-yml-file", Utils.slugify("The _config.yml file?", mode: "ascii")
+      assert_equal "the-config-yml-file",
+        Utils.slugify("The _config.yml file?", :mode => "ascii")
+      assert_equal "f-rtive-glance",
+        Utils.slugify("fürtive glance!!!!", :mode => "ascii")
     end
 
     should "only replace whitespace if mode is raw" do