Support vcr cache for hosts with explicit port

[pietdevries94]

The following test creates a vcr file that with a filename that is not viable on all platforms
// xing.com:443 - https with port

The golang sum db now returns the following.
https://sum.golang.org/lookup/github.com/mat/besticon/v3@v3.20.0

not found: create zip: besticon/testdata/xing.com:443.vcr: malformed file path "besticon/testdata/xing.com:443.vcr": invalid char ':'

This small fix replaces : with _, making sure it's still a valid file name.

My editor also complained about an unused err check, which I also cleaned up in a separate commit.

[mat]

Thanks @pietdevries94 for this fix!