[Add]: CustomValidationTests.test_partial_update

This test is to make sure that validate_<attrname> is not called when
partial=True and <attrname> is not found in .data.
[Fix]: Error with partial=True and validate_<name>

The error occurs when serializer is set with partial=True and a
field-level validation is defined on a field, for which there's no
corresponding update value in .data
Merge pull request #1112 from tamakisquare/issue-1111

Test case and fix for issue 1111