Fix method redefined warning message in mapper.rb
Fix Encoding::CompatibilityError when public path is UTF-8

In #5337 we forced the path encoding to ASCII-8BIT to prevent static
file handling from blowing up before an application has had chance to
deal with possibly invalid urls. However this has a negative side
effect of making it an incompatible encoding if the application's
public path has UTF-8 characters in it.

To work around the problem we check to see if the path has a valid
encoding once it has been unescaped. If it is not valid then we can
return early since it will not match any file anyway.

Fixes #13518