Also call #to_s on value passed to #set when dealing with file input to ensure the correct type is passed over the wire to PhantomJS.
Spec for setting a file on a file input element with Pathname.
Added an entry to the Changelog for #215.
Merge pull request #215 from mjtko/fix/upload-file-path-should-be-string

Ensure value (file path) is a String when using #set on an input node