Bug: Invoke-GraphRequest fails with BadRequest for PUT Requests

[georgend]

Original Issue

msgraph-sdk-powershell/src/Authentication/Authentication/Cmdlets/InvokeMgGraphRequest.cs

Lines 664 to 672 in 390706e

	else if (Method == GraphRequestMethod.POST)
	{
	GraphRequestSession.ContentHeaders.TryGetValue(HttpKnownHeaderNames.ContentType, out var contentType);
	if (string.IsNullOrWhiteSpace(contentType))
	{
	// Assume application/json if not set by user
	GraphRequestSession.ContentHeaders[HttpKnownHeaderNames.ContentType] =
	CoreConstants.MimeTypeNames.Application.Json;
	}

Line above only defaulted the Content-Type to application/json for POST only.

Fix
Set default Content-Type header to application/json when user has not specified -ContentType for PUT,POST,PATCH requests.

Fixes #483