Do not manually encode parameters with array syntax (fixes #89)

superagent was transforming the (already-formatted)

    type[]=type1&type[]=type2

portion of the URL into

    type[][0]=type1&type[][1]=type2

This syntax is invalid, and results in an error on the WP side.

By removing the prepareParameters method, we eliminate all of our custom
handling for array syntax, and trust that superagent will take care of it.

TODO: Add a test suite to verify this integration point.