add ssl_version unittest that works everywhere

This should fix discrepancies between OpenSSL < 1.0 and > 1.0 and also
work on Ubuntu where ssl.PROTOCOL_SSLv2 has been removed
remove superfluous request parameters
remove make_headers from pool.rst

It is already in helpers.rst wasn't built anyway.
Merge pull request #142 from t-8ch/fix_ssl_test_on_old_openssl-2

Fix ssl test on old openssl.