Client replaces single quotes with double quotes in request payload

[gliljas]

https://github.com/sendgrid/csharp-http-client/blob/master/CSharpHTTPClient/Client.cs#L311

Here, the client replaces single quotes with double quotes. That is unexpected behavior.

Single quotes are perfectly valid inside JSON string tokens.

sendgrid/sendgrid-csharp#256

[thinkingserious]

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