Make sure we pass through setting attributes to the underlying SSLContext

[davisagli]

[sethmlarson]

Need to also document where we don't have parity with SSLContext. We know we don't have parity for:

• get_ca_certs()
• cert_store_stats()

Although these may be implementable in the future? Just not in our critical path.

Another thing I'm not sure we support is renegotiation? Need to test.