Added test for channel mapper fn that uses _ underscore library

// channel mapper fn that uses _ underscore JS library
func TestChannelMapperUnderscoreLib(t *testing.T) {
        mapper, err := NewChannelMapper(`function(doc) {channel(_.first(doc.channels));}`)
        assertNoError(t, err, "Couldn't create mapper")
        res, err := mapper.callMapper(`{"channels": ["foo", "bar", "baz"]}`, `{}`, noUser)
        assertNoError(t, err, "callMapper failed")
        assert.DeepEquals(t, res.Channels, SetOf("foo"))
}