When a request includes a body, and `UnmarshalRequest` is called with
`body = true`, then the function only unmarshals the JSON body into the
target struct. However, if the target struct also includes fields for
query/path/header parameters (i.e. fields with a "lambda" struct tag),
then they were not parsed.

To fix, the request body will first be unmarshaled into the target
struct, and then the rest of the parameters (if any) will be processed
as well.