Disallow spaces in generated keys

A find call like that illustrated in the spec in this commit would
result in a key containing spaces, which is invalid for memcached with
ASCII protocol.

More sanitization might be warranted: at least newlines, carriage
returns and null bytes are invalid, I believe, but these seem unlikely
to pop up from Ruby's *args.to_s...
Merge pull request #13 from ches/strip-spaces-from-keys

Disallow spaces in generated keys