ClientOptions Dialer introduces backward compatibility issue, when used without NewClientOptions()

[GilGil1]

Hi, I'm not sure about that issue, yet it caused a working code to panic.
The issue is also ours, as we are setting the ClientOptions without using the NewClientOptions().
Members are public, therefore we are tempted to set those directly.
We didn't initiate the new member dialer, as it was not needed on last release v 1.3.5
The program crashes when trying to dial in file netconn.go:
The change of signature, enables the use of a nil dialer:
was
conn, err := tls.DialWithDialer(&net.Dialer{Timeout: timeout}, "tcp", uri.Host, tlsc)
changed to:
conn, err := tls.DialWithDialer(dialer, "tcp", uri.Host, tlsc)

suggest to add a protection at a line before to catch a nil dialer and replace it by the default one
I can assist with a PR if needed..

[MattBrittan]

Thanks @GilGil1 - as this was a quick fix I have merger PR #577 to resolve the issue.