Create a new static readonly string JsonContentType for the "application/json" string literal

Previously users wanting access to the string literal "application/json" needed to do one of the following:

* Embed the string literal `"application/json"` in their code
* Use the inefficient/obscure syntax `new JsonRequestSettings().ContentType`

This change exposes a new `static readonly` string `JsonRequestSettings.JsonContentType` for this constant, and updates string literals within the library to reference this new field instead.