Fix #1370 by making copy of default AuthMechanisms

[evolvedlight]

Proposed Changes

This fixes #1370 by making a copy of the default auth mechanisms, rather than providing a reference to the static (shared) DefaultAuthMechansisms

Types of Changes

What types of changes does your code introduce to this project?
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• [x ] Bug fix (non-breaking change which fixes issue ConnectionFactory Authmechanisms has incorrect sharing between instances #1370)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

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• [x ] I have read the CONTRIBUTING.md document
• [x ] I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• [x ] All tests pass locally with my changes
• [x ] I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

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[michaelklishin]

Thank you!

I cannot promise when the next 6.x release will go out but it should not take too long.