Add application/json header to gulp proxy if it doesn't exist

Not sure why this is needed when proxying, but it seems to be now that
we've switched to fetch based requests.

Gulpfile.js

@@ -227,6 +227,14 @@ function dev_server(done) {
                 console.log("-->", path);
                 return path;
             },
+            proxyReqOptDecorator: function (proxyReqOpts, srcReq) {
+                return new Promise(function (resolve, reject) {
+                    if (!("Content-Type" in srcReq.headers)) {
+                        proxyReqOpts.headers["Content-Type"] = "application/json";
+                    }
+                    resolve(proxyReqOpts);
+                });
+            },
         });
 
     devserver.use("/api", backend_proxy("/api"));