fix prefix_range_end

[PingHao]

to align with etcd go client implement
https://github.com/etcd-io/etcd/blob/e591fcba335ef41f23d675563bed526d2f5bd72c/client/v3/op.go#L365

prefix_range_end(b'ab\xff') should be b'ac'

[kragniz]

@tmccombs could you take a look at this pr? I'm not sure how #1125 and this fit together

[tmccombs]

This looks mostly right to me. I will note though that the behavior is slightly different than the Go implementation. This raises an exception if the prefix is entirely composed of b'\xff', whereas the go implementation returns b'\0'`, which would I believe result in an empty response.

[PingHao]

This looks mostly right to me. I will note though that the behavior is slightly different than the Go implementation. This raises an exception if the prefix is entirely composed of b'\xff', whereas the go implementation returns b'\0'`, which would I believe result in an empty response.

@tmccombs Thanks to catch it up. code updated.