Discover on Mongo shouldn't engage discovery heuristics on values

[mrocklin]

When we interact with Python data we use full heuristic mode. This does things like say that discover('1.0') == float64. This is appropriate when we guess the datashape of data in text files like CSV. This isn't appropriate when we grab data from Mongo which is already somewhat typed. In these cases a string is a string and we shouldn't do any guess work.

Perhaps the dual meaning of discover should be made more explicit.