Duplicate content type added for sendMail POST request -tst

[MIchaelMainer]

Describe the bug
Graph Explorer -tst is adding duplicate content-type value to the Content-Type header. The sample has a default value content-type header value, and then it appears that Graph Explorer is adding a second header value.

To Reproduce
Steps to reproduce the behavior:

1. Choose the **send an email ** sample.
2. Click on Run query

Expected behavior
The request should be successful. The response should not show:

{
    "error": {
        "code": "RequestBodyRead",
        "message": "The specified content type 'application/json, application/json' contains either no media type or more than one media type, which is not allowed. You must specify exactly one media type as the content type.",
        "innerError": {
            "request-id": "e42b6e3d-aec0-4e66-8146-9237bfd497d3",
            "date": "2020-05-07T23:22:42"
        }
    }
}

Additional context
If you remove the content-type header from the request headers, GE adds a content-type header. This appears to be a problem for all POST requests. It may be an issue for other HTTP methods.