4.2. Content-Range
Content-Range field 无效(invalid)的情况
A Content-Range field value is invalid if it contains a
byte-range-resp that has a last-byte-pos value less than its
first-byte-pos value, or a complete-length value less than or equal
to its last-byte-pos value. The recipient of an invalid
Content-Range MUST NOT attempt to recombine the received content with
a stored representation.
说的就是当如下3种情况时Content-Range无效，这里用bytes-units的情况举例:
Content-Range: bytes 45-44/1234
Content-Range: bytes 45-44/44
Content-Range: bytes 45-44/43
当recipient判断出"invalid"时就不能执行recombine。
上述“判断”显然只是“recipient的本地判断”，recipient根据上述判得出的并不是最“精确”的结果，毕竟如果服务器可能会给一个通过上述判断结果为"valid",但实际“错误”的Content-Range，例如:
representation实际长度1000
//服务器返回
Content-Range: bytes 45-44/300
显然，服务器返回了一个错误的结果。
