fix some quick wins

[bollhals]

Proposed Changes

Fixes 2 easy cases of allocations.

1. GC.SuppressFinalize(this); for a struct is useless and boxes the struct allocating the box.
2. BasicPublishMemory is only useful if you already have a memory, encoding it on the fly is worse than using BasicPublish instead.

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• 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)
• Performance change

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• 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

[michaelklishin]

@bollhals thank you!