Updated from 0.3.0 to 0.3.7 and pem files with certs and keys no longer work with warp tls

[jdthomas]

Version
Worked in 0.3.0 fails in 0.3.7.

cat /path/to/my/server.pem | grep 'BEGIN\|END'
-----BEGIN CERTIFICATE-----
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
-----END CERTIFICATE-----
-----BEGIN PRIVATE KEY-----
-----END PRIVATE KEY-----

Platform
N/A

Description
Usage looks roughly like:

    let (https_addr, https_server) = warp::serve(routes)
        .tls()
        .cert_path(&cert_path)
        .key_path(&cert_path) // key and cert are in same file in .pem files
        .bind_with_graceful_shutdown((addr, *port), async move {
            // ...
        })

In the old code the builder roughly did: "give me the pkcs8 private key" nope, give me the "rsa private key" ok/nope ...

In the 3.7 code it checks that all of the entries in the pem file are keys before using the last entry.

Will post a PR to ignore non-key entries shortly and begin a conversation on the correct fix ...