Empty/NotEmpty return confusing results for custom types with underlying type string

[bits01]

Consider the following example:

type key string

func TestEmpty(t *testing.T) {
	var k key = ""
	assert.Empty(t, k, "confusing to fail")
	assert.NotEmpty(t, k, "this is very error prone when used in tests")
}

The assert.Empty above fails. While I understand what is going on, it is confusing and very error prone in tests when one uses assert.NotEmpty instead.

[bits01]

Please take a look at PR #470.

reflect.Value.Len() can cover more types, per godoc:
https://golang.org/pkg/reflect/#Value.Len

Len returns v's length. It panics if v's Kind is not Array, Chan, Map, Slice, or String.

[bits01]

PR #470 merged. Thank you.