change the field
Merge pull request #234 from edx/danielli/pyfs

We use fields to generate keys. This fix makes this work in edx-platform, where these are not strings. 

By abstraction, we should generally not rely on this being "stringy," but in this case, we went through this with Cale, and it seems like the least of all evils. Otherwise, we'd replicate a lot of code.