Fix BeNilMatcher not matching nil pointers

[pjvds]

I noticed that the BeNilMatcher returns false for pointers that point to a nil value. Here is a code example that fails:

var _ = Describe("BeNil", func() {
    It("should succeed when passing nil pointer", func() {
        var f *struct{}
        Ω(f).Should(BeNil())
    })

This is because actual is used as an interface{} type inside the BeNilMatcher. I have changed the implementation of it to, when the actual == nil check fails, it uses reflection to get check if actual is a pointer and if so, it check the element value for nil.

Here is the implementation:

func (matcher *BeNilMatcher) Match(actual interface{}) (success bool, message string, err error) {
    if actual == nil {
        return true, formatMessage(actual, "not to be nil"), nil
    } else {
        // When the value is not nil, check if it is a pointer
        // value and check if the element is nil.
        t := reflect.ValueOf(actual)
        if t.Kind() == reflect.Ptr {
            return t.IsNil(), formatMessage(actual, "to be nil"), nil
        }

        return false, formatMessage(actual, "to be nil"), nil
    }
}

[onsi]

great catch. thanks for the PR!