change ref to in and use Unsafe.AsRef

[bollhals]

Proposed Changes

This change avoids having to expose the ref BasicProperties parameter we have introduced when switching over to the struct BasicProperties.

Ref has 2 downsides,

• 1. it's not readonly, so the user could think we're editing the struct, which we do not want to do.
• 2. you have to put ref infront of it everywhere

This PR changes it to be in, and in the lowest possible place use Unsafe.AsRef to cheat our way to be able to use ref for the SerializeToFrames method.
(Which requires it, as the generic method creates defensive copies of the struct if it is n for each method call, as it is not aware that the method itself is readonly due to the interface description not allowing to mark a method as readonly. (This is a dotnet limitation))

So with this we get the best for the user and an acceptable workaround within the library.

Undos commit af8c072 of #1096

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)

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

[lukebakken]

Thank you!