Replace merge_paths with Path::Tiny:

This was a bit hard to figure out, but with the help of @wchristian,
it seems as though this merges path from argument 1 onto argument 2.

I have this replaced it with a single path() call. All tests pass,
so I hope this is okay. We should run these tests under Windows to
make sure it works there as well.

lib/Dancer2/Core/App.pm

@@ -1019,7 +1019,7 @@ sub send_file {
                 || $self->config->{public_dir}
                 || Path::Tiny::path( $self->location, 'public' )->stringify;
 
-        $file_path = Dancer2::Handler::File->merge_paths( $path, $dir );
+        $file_path = Path::Tiny::path( $dir, $path )->stringify;
         my $err_response = sub {
             my $status = shift;
             $self->response->status($status);

lib/Dancer2/Handler/File.pm

@@ -85,7 +85,7 @@ sub code {
             $path =~ s/^\Q$prefix\E//;
         }
 
-        my $file_path = $self->merge_paths( $path, $self->public_dir );
+        my $file_path = Path::Tiny::path( $self->public_dir, $path )->stringify;
         return $self->standard_response( $app, 403 ) if !defined $file_path;
 
         if ( !-f $file_path ) {
@@ -130,22 +130,4 @@ sub code {
     };
 }
 
-sub merge_paths {
-    my ( undef, $path, $public_dir ) = @_;
-
-    my ( $volume, $dirs, $file ) = File::Spec->splitpath( $path );
-    my @tokens = File::Spec->splitdir( "$dirs$file" );
-    my $updir = File::Spec->updir;
-    return if grep $_ eq $updir, @tokens;
-
-    my ( $pub_vol, $pub_dirs, $pub_file ) = File::Spec->splitpath( $public_dir );
-    my @pub_tokens = File::Spec->splitdir( "$pub_dirs$pub_file" );
-    return if length $volume and length $pub_vol and $volume ne $pub_vol;
-
-    my @final_vol = ( length $pub_vol ? $pub_vol : length $volume ? $volume : () );
-    my @file_path = ( @final_vol, @pub_tokens, @tokens );
-    my $file_path = path( @file_path );
-    return $file_path;
-}
-
 1;