NuGet package updates

What's Changed

• Improve encrypt tests by @mattosaurus in #272
• Add recipient tests by @mattosaurus in #273
• Add sign tests by @mattosaurus in #275
• Add encrypt header test by @mattosaurus in #278
• Add verify tests by @mattosaurus in #279
• Update README.md with more Inspect method details by @mattosaurus in #282
• Minor correction of XML comments and method renaming (UseEncrytionKey to UseEncryptionKey) by @bizouarn in #283
• Chore/package updates by @mattosaurus in #290

Full Changelog: v6.4.0...v6.4.1