Feature Request

Motivation

The User-Agent header is populated, by default, with tonic/x.y.z (x.y.z. being the current tonic version number)
There is a user_agent method to somehow customize this by adding a prefix to this tonic/x.y.z part.

Proposal

In some use cases, users could want to totally customize the user-agent. Not suffixing with tonic/x.y.z would probably be a good idea.

I'm not sure why this suffix has been made compulsory anyway (but maybe I'm missing something? Is this required by some spec? I feel that any server requiring to know the client version is probably poorly designed)

Thanks