Add support for optional parts in http request api #14
Description
{:body
{:base "part"}
:optional-body-parts
{pred-expr0 {:foo "bar"}
pred-expr1 [{:fizz "buzz"} {:quux 1}]}}{{ message }}
{:body
{:base "part"}
:optional-body-parts
{pred-expr0 {:foo "bar"}
pred-expr1 [{:fizz "buzz"} {:quux 1}]}}