Client side certificate support

[Magua]

Client certificate supported with PKSC 12 / pfx.

Use like this for password-less p12.

requests.get(
  "https://client.badssl.com",
  cert = "./badssl.com-client.p12"
)

Add password if necessary.

requests.get(
  "https://client.badssl.com",
  cert = ("./badssl.com-client.p12", "password")
)

badssl.com used in tests can and will occasionally (annually?) update certificates and that would fail the tests. Therefore I made the test print a warning in case of client certificate failure instead of failing the test.

Warning looks like this:

WARNING: Certificate may have expired and needs to be updated. Please check: https://github.com/lihaoyi/requests-scala/blob/master/requests/test/resources/badssl.com-client.md and/or file issue

[Magua]

Closes #12

[lionelfleury]

Nice work! 👍

[Magua]

@lihaoyi have you had a chance to look at this, anything you want done differently?

[lihaoyi]

Sorry, i just haven't had a chance to take a look at this properly. I'll try to find time for a proper review soon!

[lihaoyi]

Looks good to me, sorry for the delay!