move to deep validator for TableInfo indexed attributes

Author: mattsb42-aws

src/dynamodb_encryption_sdk/structures.py

@@ -17,7 +17,7 @@
 import six
 
 from .identifiers import ItemAction
-from dynamodb_encryption_sdk.internal.validators import dictionary_validator
+from dynamodb_encryption_sdk.internal.validators import dictionary_validator, iterable_validator
 
 
 def _validate_attribute_values_are_ddb_items(instance, attribute, value):
@@ -176,7 +176,7 @@ class TableInfo(object):
         default=None
     )
     _indexed_attributes = attr.ib(
-        validator=attr.validators.optional(attr.validators.instance_of(set)),
+        validator=attr.validators.optional(iterable_validator(set, six.string_types)),
         default=None
     )