Prevent nil pointer dereference

[mvrahden]

Contains*-Assertions panic when a nil instead of a string, slice, map etc. is injected.
Minimal example here:

var myMap map[string]interface{} // empty
// try an assertion on nested keys
// which effectively leads to a passing of nil
require.Contains(t, myMap["some-key"], "expected-nested-key") // <-- panic

Assertions should be robust against nil-injection and gracefully fail instead of panicking.