Fix download strategy for SVN HEAD

After converting to Pathname to create paths, using '+=' will result in
path concatenation by '/', which result in weird path like
'gcc--svn/-HEAD'. This patch should fix this.

Closes Homebrew/homebrew#19233.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/download_strategy.rb

@@ -217,7 +217,7 @@ class SubversionDownloadStrategy < AbstractDownloadStrategy
       @co = Pathname.new("#{HOMEBREW_CACHE}/#{name}--svn")
     end
 
-    @co += "-HEAD" if ARGV.build_head?
+    @co = Pathname.new(@co.to_s + '-HEAD') if ARGV.build_head?
   end
 
   def cached_location