Added test for Element's custom attributes

Based on a discussion in [1], this tests ensures that Element's dataset
properties does not interfere with namespaced attributes with the same
name, but in different namespaces (in the former case, all Element's
dataset properties are stored as attributes with no namespace).

[1] https://critic.hoppipolla.co.uk/showcomment?chain=8907
Merge pull request #1440 from brunoabinader/custom-attrs

Added test for Element's custom attributes; r=Ms2ger