Use xsd:dateTime format (ISO 8601) for NOW

[RubenVerborgh]

Fixes https://github.com/comunica/sparqlee/issues/59

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Pull Request Test Coverage Report for Build 340

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 79.331%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/functions/SpecialFunctions.ts	1	2	50.0%
<!--	Total:	1	2	50.0%	-->

Totals
Change from base Build 338:	0.0%
Covered Lines:	1046
Relevant Lines:	1266

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💛 - Coveralls