fix SSL issue in proxy

when fetching diagrams from SSL Endpoints, handshake may fail in java >= 1.7 with
"javax.net.ssl.SSLProtocolException: handshake alert: unrecognized_name"

this patch disables SNI Extension. proxy now works for https://raw.githubusercontent.com/some/privatepage?login=user%26token=some2143token

it is a quick fix to show the issue and hint for the root cause. 
please advise how this should be implemented.