fixes wrong path detection when url ends with /
fixes wrong path detection when url ends with /

Trailing `/` causes `Path.GetDirectoryName` to assume last path as directory instead of file.


Following code demonstrates the issue.

```csharp
void Main()
{
	const string url = "https://docs.microsoft.com/en-us/aspnet/mvc/videos/";

	/*
	PATH
	C:\temp\cache\docs.microsoft.com\en-us\aspnet\mvc\videos 

	FILENAME

	*/

	Parse(url);

	/*
	PATH
	C:\temp\cache\docs.microsoft.com\en-us\aspnet\mvc 

	FILENAME
	videos
	*/

	Parse(url.TrimEnd(Path.AltDirectorySeparatorChar));
}

private void Parse(string url)
{
	const string defaultExtension = ".html";
	const string cachePath = @"C:\temp\cache\";
	
	var uri = new Uri(url);
	bool isAbsolutePathRoot = uri.AbsolutePath.Equals(Path.AltDirectorySeparatorChar.ToString());
	var suffix = (uri.Host + uri.AbsolutePath).Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar);
	string path = Path.Combine(cachePath, isAbsolutePathRoot
														 ? defaultExtension
														 : suffix);
	Path.GetDirectoryName(path).Dump("PATH");
	Path.GetFileName(path).Dump("FILENAME");
}
```
zzzprojects/html-agility-pack,zzzprojects/html-agility-pack