Hashing fails when fed a directory

[Mossop]

A straightforward glob that matches all files in a directory also ends up matching directories that this plugin then attempts and fails to hash.

Example code:

return gulp.src(path.join(__dirname, "static", "client", "**", "*"))
    .pipe(hash())
    .pipe(gulp.dest(path.join(__dirname, "build", "static")));

Result:

TypeError [ERR_INVALID_ARG_TYPE]: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received null
    at Hash.update (internal/crypto/hash.js:84:11)
    at generateHash (/Users/dave/workspace/pixelbin/node_modules/gulp-hash-filename/lib/generateHash.js:5:8)
    at performHash (/Users/dave/workspace/pixelbin/node_modules/gulp-hash-filename/lib/performHash.js:13:13)
    at Transform.assemblyStream._transform (/Users/dave/workspace/pixelbin/node_modules/gulp-hash-filename/index.js:14:15)
    at Transform._read (_stream_transform.js:205:10)
    at Transform._write (_stream_transform.js:193:12)
    at writeOrBuffer (_stream_writable.js:352:12)
    at Transform.Writable.write (_stream_writable.js:303:10)
    at Transform.ondata (/Users/dave/workspace/pixelbin/node_modules/through2/node_modules/readable-stream/lib/_stream_readable.js:681:20)
    at Transform.emit (events.js:314:20)

I can of course fix this by setting the nodir option on gulp.src but it might be good for this plugin to just skip over directories by default.