Cannot download a string variable through the Flow interface

[curtislisle]

An analysis was created which writes out a string. The string is viewable through the "visualization tab", but no type is available when trying to download it. Downloading without a type generates a runtime error: "http://localhost:8080/api/v1/item/flow/string/text/null" since no type was available to pick from.

[danlamanna]

This should be fixed by girder/girder_worker#23.