Steps to reproduce:

1. Create httptest with tags.
2. Check tag existance of relevant httptest items in item_tag table. Observe: Httptest tags are propagated to all items.
3. Send request to httptest.update API method without passing tags parameter but passing steps parameter.
4. Check tag existance of relevant httptest items in item_tag table. Observe: All tags were deleted for all items.
5. Repeat first two steps.
6. Send request to httptest.update API method without passing both, tags and steps parameters.
7. Check tag existance of relevant httptest items in item_tag table. Observe: All tags were deleted only for directly related items of httptest. Step items still have tags.

Result:
Tags are unexpectedly deleted from httptest items.
Expected:
Httptest item tags remain to exist.

NB: The issue is reproducible only in 6.0. The optional DB upgrade patches should be prepared for 7.0, 7.4 versions, and mandatory DB upgrade patch for 8.0.