Add required user-agent to crates-io lib api (breaking change)

[FGRibreau]

Since cargo.io now requires a User-Agent header (see message below), I've updated the crates-io crate accordingly. Hope this will be approved and generate a new release of crates-io :)

This is a breaking change because there is an contract change on (added user_agent: Option<String>):

• Registry::new(host: String, token: Option<String>, user_agent: Option<String>)
• Registry::new_handle(host: String, token: Option<String>, user_agent: Option<String>, handle: Easy)

Message from crates.io:

We require that all requests include a `User-Agent` header. 
To allow us to determine the impact your bot has on our service, we ask that your user agent actually identify your bot, and not just report the HTTP client library you're using.  
Including contact information will also reduce the chance that we will need to take action against your bot.

Bad:
User-Agent: reqwest/0.9.1

Better:
User-Agent: my_crawler

Best:
User-Agent: my_crawler (my_crawler.com/info)
User-Agent: my_crawler (help@my_crawler.com)

If you believe you've received this message in error, please email help@crates.io and include the request id 7da610ae-264a-4738-a979-c53465fa8c99.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ehuss (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[ehuss]

Thanks, I ran into this with another project.

src/cargo/ops/registry.rs will need to be updated to no longer add the user-agent manually.

Also, I'm thinking it might be better to not use an Option, and require an explicit value.

[FGRibreau]

Also, I'm thinking it might be better to not use an Option, and require an explicit value.

@ehuss you're right indeed, I removed the Option :)

[ehuss]

Can you also update src/cargo/ops/registry.rs as mentioned above?

[ehuss]

Hm, I think I wasn't clear. There is custom handling for the user agent in configure_http_handle. That will need to be handled, and somehow shared. Unfortunately this stuff is used all over, so it may not be so simple to update. Perhaps it would be best just to update Registry::new and leave Registry::new_handle unchanged? Cargo doesn't use Registry::new, and maybe a doc comment could be added to new_handle informing the user they must set the user-agent header.

[bors]

☔ The latest upstream changes (presumably #7011) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

This has gone stale for some time now so I'm going to close this, but feel free to resubmit of course!