DirectoryNotFound is not Thrown by MockFile Create #278

I had added a test to my project where I wanted to make sure that my code would create the directory for an output file before writing the output file, this test passed, but when I went to run my code for real, I got a DirectoryNotFound exception. It turns out in my implementation, I had neglected to create the directory. When I investigated why my test reported that the directory was created, when there was no code to create the directory, I found the following.

my code gets an output stream by calling:

IFileSystem.File.Open(outputFilename, FileMode.Create, FileAccess.Write, FileShare.None);

When the test is running this calls System.IO.Abstractions.TestingHelpers.MockFile.Open.

In the MockFile implementation of open, it eventually calls:

if (mode == FileMode.Create || mode == FileMode.Truncate)
{
    Delete(path);
    return Create(path);
}

Looking at the Create function:

public override Stream Create(string path)
{
    if (path == null)
    {
        throw new ArgumentNullException("path", "Path cannot be null.");
    }
    mockFileDataAccessor.PathVerifier.IsLegalAbsoluteOrRelative(path, "path");
    mockFileDataAccessor.AddFile(path, new MockFileData(new byte[0]));
    var stream = OpenWrite(path);
    return stream;
}

We see that this code doesn't check that the directory part of the path parameter exists. When AddFile is called, the directory gets added to the mock file systems known directories. I think this code should throw a DirectoryNotFound exception when the directory does not exist.

This appears to be similar to the issue described here:
#140