Lower allocations when building nav

This cuts down various string allocations in nav helpers,
which are used a lot and inside of loops. Minor performance
improvement.

app/helpers/application_helper.rb

@@ -44,7 +44,7 @@ module ApplicationHelper
   #   current_controller?('gitlab/application') # => false
   def current_controller?(*args)
     args.any? do |v|
-      v.to_s.downcase == controller.controller_name || v.to_s.downcase == controller.controller_path
+      safe_downcase!(v.to_s) == controller.controller_name || safe_downcase!(v.to_s) == controller.controller_path
     end
   end
 
@@ -59,7 +59,7 @@ module ApplicationHelper
   #   current_action?(:create)        # => false
   #   current_action?(:new, :create)  # => true
   def current_action?(*args)
-    args.any? { |v| v.to_s.downcase == action_name }
+    args.any? { |v| safe_downcase!(v.to_s) == action_name }
   end
 
   def admin_section?
@@ -397,6 +397,19 @@ module ApplicationHelper
     end
   end
 
+  # A safe alternative to String#downcase!
+  #
+  # This will make copies of frozen strings but downcase unfrozen
+  # strings in place, reducing allocations.
+
+  def safe_downcase!(str)
+    if str.frozen?
+      str.downcase
+    else
+      str.downcase! || str
+    end
+  end
+
   private
 
   def appearance

app/helpers/tab_helper.rb

@@ -72,7 +72,8 @@ module TabHelper
     # Add our custom class into the html_options, which may or may not exist
     # and which may or may not already have a :class key
     o = options.delete(:html_options) || {}
-    o[:class] = [*o[:class], klass].join(' ').strip
+    o[:class] = [*o[:class], klass].join(' ')
+    o[:class].strip!
 
     if block_given?
       content_tag(:li, capture(&block), o)