Don't append slice if not asked to

[vdemeester]

Fix the case when we don't ask for slice to be appended but are
because of src and dst values.

cc @imdario seems like this is the cause of the build failure here

09:18:39 --- FAIL: TestLoadDaemonConfigWithNetwork (0.00s)
09:18:39 	daemon_unix_test.go:46: assertion failed: 127.0.0.1 (string) != ?00000000000000000000ffff7f00000100000000000000000000ffff00000000 (string)

I'm not a huge fan of that change as it's probably going to break current usage : a slice in a map is always appending, with this change it will only be appended if it is asked to

~/s/g/i/mergo{slice-merge-fix} λ go test ./...
ok      github.com/imdario/mergo        0.002s

Signed-off-by: Vincent Demeester vincent@sbr.pm

[darccio]

Done!