Come up with a pattern for handling POST parameters

[markandrus]

When you POST to a Twilio resource, sometimes your POST a subset of the parameters on the resource itself, and sometimes you POST parameters not present on the resource itself. A few questions:

• Should we represent this as a separate data type?
• Should we use a type family from GET resources to POST data types so that we can write a general POST function?

Records in Haskell make this more annoying than it should be.