Fix frozen string literal issue

Author: chaadow

lib/fog/aws/signaturev4.rb

@@ -109,7 +109,7 @@ def canonical_headers(headers)
         canonical_headers = ''
 
         for key in headers.keys.sort_by {|k| k.to_s.downcase}
-          canonical_headers << "#{key.to_s.downcase}:#{headers[key].to_s.strip}\n"
+          canonical_headers += "#{key.to_s.downcase}:#{headers[key].to_s.strip}\n"
         end
         canonical_headers
       end