*  5. Open a source to read the response body. Either [fixed-length][newFixedLengthSource],
 *     [chunked][newChunkedSource] or [unknown][newUnknownLengthSource].
 *  6. Read from and close that source.
 *
 * Exchanges that do not have a request body may skip creating and closing the request body.
 * Exchanges that do not have a response body can call
 * [newFixedLengthSource(0)][newFixedLengthSource] and may skip reading and closing that source.
 */