Fix lifetime in ElementRef::attr return value

[g2p]

I tried to used the new accessor (from #140) on release and fixed a gotcha.

This allows returning references to the document when the traversal is done in an inner function.

[cfvescovo]

Good catch

[adamreichold]

I am running into this as well now that I want to use this new helper method. A 0.18.1 patch release to fix this would be great. (I think this should be backwards compatible as the lifetime is strictly longer.)

[cfvescovo]

I am releasing the patch

[adamreichold]

Thank you for investing your time to make the fix available!