Fixes AddCustomAttribute throws Newtonsoft.Json.JsonWriterException: Unsupported type error when used to add header values

[jaffinito]

Resolves #377

Description

Adds an if statement to JsonSerializerHelpers.cs to check if this the type is Microsoft.Extensions.Primitives.StringValues.
Went with strings since VS was not happy with using just the types as the conditional.

Added a small unit test to AgentApiTests.cs that checks StringValues.

Testing

Tests should pass as normal.

Changelog

Fixes a bug where using AddCustomAttribute throws Newtonsoft.Json.JsonWriterException: Unsupported type error when used to add header values.

[nr-ahemsath]

Looks good to me. Thanks for adding a test.

If we had more time, I would like to explore a more generic fix for this issue, but we want to get this fix into today's release so we'll go with this as-is.

[vuqtran88]

LGTM. We might want to find a better way to address it generically in the future.