Only allow one value on certain headers

[edouardpoitras]

This is related to the bug I submitted on msgraph-sdk-dotnet: microsoftgraph/msgraph-sdk-dotnet#1951

[andrueastman]

Thanks for the contribution here @edouardpoitras. Really do appreciate it.

Apart from the comments, can you also please update the following before we merge this?

• Bump the version at https://github.com/microsoft/kiota-abstractions-dotnet/blob/main/src/Microsoft.Kiota.Abstractions.csproj#L16
• Update the Changelog file

[baywet]

Thanks for the contribution!
Aside from Andrew's comments, I wanted to provide more context here Content-Type is a singleton field

Although Content-Type is defined as a singleton field, it is sometimes incorrectly generated multiple times, resulting in a combined field value that appears to be a list

See more information about how fields should be folded.

Looking at the other metadata fields it sounds to me that the following fields should also be added to this list:

• Content-Encoding
• Content-Length
• Content-Location (only applies to responses, maybe skipped)
• Last-Modified (same)

[baywet]

Thanks for making the changes. @andrueastman for final review and merge.

[andrueastman]

Thanks again @edouardpoitras