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As always, fix compilation with Visual Studio because of variable not declared at the beginning of a block...

rfc2617: The values of the opaque and algorithm fields must be those supplied in WWW-Authenticate response header

Fix nonce reutilization test: since everything is not cleared before each test, compare with previous value instead of 0

The object-oriented architecture was not used correctly for the TLS channel. The call to belle_sip_channel_set_socket was reinitializing the belle_sip_source_t and so erasing the timeout value initialized by stream_channel_connect.
The fix is to just replace the notify callback (that was the only goal of the call to belle_sip_channel_set_socket).
Also added a unit test to check this.