leading slash leads to unnecessary  redirects

Hi,

we have 2 issues with keys that have a leading slash:

- They lead to unnecessary redirects, where the double slash is reduced to a single slash (e.g. [nioEventLoopGroup-2-2] WARN mousio.etcd4j.transport.EtcdKeyResponseHandler - redirect for /v2/keys//etcd4jTestDir/etcd4jTest?value=test to /v2/keys/etcd4jTestDir/etcd4jTest?value=test)
- In our setup a put with a leading slash silently fails... no exception, nothing special in the logs, only the value is not stored in etcd. Unfortunately I could not reproduce this in a test class...

So I just added a check for a leading slash in setKey() of EtcdKeyRequest. No redirects anymore, and in our setup it works, too.

Greetings, Marc