extract url escaping to a dedicated class method and improved tests

lib/final_destination.rb

@@ -10,9 +10,10 @@ class FinalDestination
   attr_reader :cookie
 
   def initialize(url, opts = nil)
+    @url = url
     @uri =
       begin
-        URI(URI.escape(CGI.unescapeHTML(url), Regexp.new("[^#{URI::PATTERN::UNRESERVED}#{URI::PATTERN::RESERVED}#]"))) if url
+        URI(escape_url) if @url
       rescue URI::InvalidURIError
       end
 
@@ -176,6 +177,10 @@ class FinalDestination
     false
   end
 
+  def escape_url
+    URI.escape(CGI.unescapeHTML(@url), Regexp.new("[^#{URI::PATTERN::UNRESERVED}#{URI::PATTERN::RESERVED}#]"))
+  end
+
   def private_ranges
     FinalDestination.standard_private_ranges +
       SiteSetting.blacklist_ip_blocks.split('|').map { |r| IPAddr.new(r) rescue nil }.compact