Rustls support no longer works with TLS 1.2 connections

[charlespierce]

With the recent upgrade of rustls in #175, the default features were correctly turned off to enable the ring cryptography provider. However, in the process, the default feature tls12 was not re-enabled. That feature adds support for TLS 1.2 connections, so at the moment using attohttpc with rustls we see a ProtocolVersion error when attempting to fetch from registry.npmjs.org. (See this CI failure)

Testing locally, it appears that re-enabling the tls12 feature solves the underlying issue.