replace strings with symbols

lib/paperclip/rails_environment.rb

@@ -15,7 +15,7 @@ module Paperclip
     private
 
     def rails_exists?
-      Object.const_defined?("Rails")
+      Object.const_defined?(:Rails)
     end
 
     def rails_environment_exists?

lib/paperclip/storage/s3.rb

@@ -286,7 +286,7 @@ module Paperclip
       end
 
       def parse_credentials creds
-        creds = creds.respond_to?('call') ? creds.call(self) : creds
+        creds = creds.respond_to?(:call) ? creds.call(self) : creds
         creds = find_credentials(creds).stringify_keys
         (creds[RailsEnvironment.get] || creds).symbolize_keys
       end