fixed NullpointerException in content type during post call

Author: krototype

src/main/java/com/microsoft/graph/http/CoreHttpProvider.java

@@ -288,6 +288,7 @@ public Request authenticateRequest(Request request) {
 					// This ensures that the Content-Length header is properly set
 					if (request.getHttpMethod() == HttpMethod.POST) {
 						bytesToWrite = new byte[0];
+						contenttype = Constants.BINARY_CONTENT_TYPE;
 					}
 					else {
 						bytesToWrite = null;