Don't set the transport's proxy settings to nil by default

[neilotoole]

In client.go, at this location, there's this code:

    if req.proxyURL != nil {
        c.transport.Proxy = http.ProxyURL(req.proxyURL)
    } else if c.proxyURL != nil {
        c.transport.Proxy = http.ProxyURL(c.proxyURL)
    } else {
        c.transport.Proxy = nil
    }

The final else clause should be removed. If the caller provides a transport, it may already have a proxy configured, there's no reason to set the transport's proxy to nil.

[jeevatkm]

Make sense. Yes, we can leave that to caller.

Would you like to send PR?

[jeevatkm]

Taken care.