Add option for IPv6 agent to request node

[JKRhb]

This PR is a potential fix for #15, adding an option to the request node for using a globalAgentIPv6 in the underlying node-coap instead of its IPv4 counterpart.

Once it is merged, the PR may fix #15. More testing and investigation is required, however.

[coveralls]

Pull Request Test Coverage Report for Build 584592034

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.9%) to 89.944%

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Files with Coverage Reduction	New Missed Lines	%
coap/coap-request.js	4	91.92%

Totals
Change from base Build 584381314:	-0.9%
Covered Lines:	116
Relevant Lines:	126

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💛 - Coveralls