Bytecode silently drops the fetch body argument #143

Found during #142. The bytecode compiler silently drops the third (body) argument of (fetch url method body). Making it fail closed breaks TestProdFlipCriteriaLock/fetch_body and the nested_multi_effect_* conformance cases, which lock in "the body is skipped". Changing that is a decision about the flip criteria and needs the Owner's sign-off.