Change GraphProxy.java contentType condition to prevent re-encoding of response (#678)

AzureAD Graph API has changed the MIME Type like `application/json;odata.metadata=minimal;odata.streaming=true;IEEE754Compatible=false`, which is `String#equals` cannot cover.

Author: rebel1324

src/main/java/com/microsoft/jenkins/azuread/GraphProxy.java

@@ -146,7 +146,7 @@ private void proxy(StaplerRequest2 request, StaplerResponse2 response) throws IO
             response.addHeader("client-request-id", okResp.header("client-request-id"));
             ResponseBody body = okResp.body();
             if (body != null) {
-                if ("application/json".equals(contentType)) {
+                if (contentType.startsWith("application/json")) {
                     String string = body.string();
                     response.getWriter().write(string);
                 } else {