Regression in StaticFileHandler: ignoring {"path": …}

A regression was introduced in 3f04edf5546e388293d8d1d2f015ca74f8a93217 which causes the StaticFileHandler to ignore the `{"path": …}` argument.

This test case passes against the previous revision, 25506060a683d2c0503e8f89c2338e8b710600fe.

I'll be able to work on a fix in the next couple of days, if you'd rather wait for one.