It might happen that CSV files will omit redundant commas, in which case d3.autoType breaks with
TypeError: Cannot read property 'trim' of undefined

This might be a desirable feature, in which case it would be good to trap the error and make it explicit.

However my personal preference would be to have it a bit more fault tolerant and avoid crashing on d3.autoType({ foo: undefined }).

Especially if d3.autoType is made the default for d3.csv (#43).