rust/protocol: allow extra whitespace between keys in get request

[brayniac]

Describe the bug
Twemcache request parsing allows for extra whitespace in some parts of a get request. While the rust protocol crate allows trailing whitespace before the CRLF, it does not allow double-spacing in a get request.

To Reproduce
Send: get 0\r\n (note two spaces) to an empty cache

Expected behavior
The expected response would be END\r\n, instead the server hangs up.