deleting elements using attribute indexing yields counterintuitive results

[josca42]

If you initialise an array with the below code

da = DocumentArray()

for test_tag in ["a", "a", "b"]:
    d = Document(tags={"test_tag": test_tag})
    da.append(d)

Then running

del da[0]
da[:, "tags__test_tag"]

Gives ["a", "b"] as expected

But if you instead do

del da[np.array(da[:, "tags__test_tag"]) == "a"]
da[:, "tags__test_tag"]

Then you get ["a", "a"] but one would expect to get ["b"]

[davidbp]

Thanks a lot @josca42 for finding this bug, we've found the issue and will patch it really soon.