fix: fix IPv6 related multicast problem

[JKRhb]

I noticed that there is a problem with IPv6 multicast addresses in the server right now. This PR adds a test case to illustrate the problem and a workaround to fix it, as iterating over all IPv6 interfaces does not work as it does in the IPv4 case.

The new test is skipped in the case of Mac OS as there seem to be problems related to IPv6 multicast (addMembership EADDRNOTAVAIL).

[coveralls]

Pull Request Test Coverage Report for Build 2213445822

• 36 of 40 (90.0%) changed or added relevant lines in 2 files are covered.
• 97 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+7.3%) to 94.533%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
test/server.ts	31	35	88.57%

Files with Coverage Reduction	New Missed Lines	%
lib/outgoing_message.ts	1	97.92%
index.ts	2	93.43%
lib/helpers.ts	2	97.07%
lib/observe_read_stream.ts	2	92.21%
lib/observe_write_stream.ts	2	95.1%
lib/cache.ts	3	92.0%
lib/segmentation.ts	3	89.58%
lib/option_converter.ts	4	94.39%
lib/middlewares.ts	5	87.4%
lib/agent.ts	28	87.27%

Totals
Change from base Build 2201295132:	7.3%
Covered Lines:	8681
Relevant Lines:	9085

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💛 - Coveralls