Add Json Content Type when create a request with body

[pvivera]

By default when you add a body to a request, a header with Content-Type and application/json is added too.

[subnetmarco]

What if the user wants to submit a body that's not a JSON body but, for example, binary?

[pvivera]

Yes, it's a possibility but regarding this code https://github.com/Mashape/unirest-net/blob/master/unirest-net/unirest-net/src/request/HttpRequest.cs#L184-187 every time that you add an object in the body is serialized as JSON.

[subnetmarco]

@pvivera this is definitely a bug, thanks for pointing it out. The client should allow an arbitrary body to be sent, without serializing it to JSON first.

[subnetmarco]

I've noticed there were some major updates to the library, I didn't have the chance to test the library, but I don't see any automatic JSON encoding in the code anymore.

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