Add AddJsonBody overload to serialise top-level string

[alexeyzimarev]

Description

Proposal to fix#1954.

Add a AddJsonBody overload with string payload argument with forceSerialize argument. If forceSerialize is set to true, the string will be serialised. If it's set to false, the string will be sent as-is.

Purpose

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

It's not a strictly breaking change, I think it might be released a s a minor version.

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[d-a-s]

I'd recommend you consider adding this as a breaking change in the release notes.

I recently upgraded an old project that was relying on the previous top level string behavior (encoding by default).