Set SNI to Host header in HostHeaderSSLAdapter

[qistoph]

Modern TLS/SSL servers can serve content and certificates based on the Server Name Indication (SNI) that is sent in the Client Hello. By setting the server_hostname in the connection pool arguments, the underlying libraries will send the provided hostname as SNI. This allows us to use the HostHeaderSSLAdapter to request content from a specific IP address, while still requesting the server to sent content based on a hostname.

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[qistoph]

Just noticed another PR (https://github.com/requests/toolbelt/pull/278) mentioning the same issue.

Though I think my contribution is a little more complete (doc update and safety setting on line 43).

[half-duplex]

Fixes #159