Send and Sync in return type of Error::get_ref

[dtolnay]

reqwest::Error::get_ref returns Option<&(std::error::Error + 'static)>.

In accordance with this API guideline, we like for error types to be Send and Sync. This is less important (or maybe not important) for references to trait objects, but here there doesn't seem to be a reason to want the flexibility of returning a non-Send or -Sync type in the future.

Returning Option<&(std::error::Error + Send + Sync + 'static)> would be consistent with the signature of std::io::Error::get_ref.