PKCS12 client certificates

Recently I had to connect to a remote service using a password protected private key file.

The `:pem` option didn't seem to allow me to generate an `OpenSSL::X509::Certificate` and an `OpenSSL::PKey::RSA` in the right way, although I was able to manually build an HTTP connection using `OpenSSL::PKCS12.new`.

In the end I extended `ConnectionAdapter` to get things working in my project, but as PKCS12 is a standard I thought I'd merge my fix back into the main `ConnectionAdapter` class and let you take a look.