Allow unsetting headers by passing null (#382) (#1845)

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/core/dispatchRequest.js

@@ -47,6 +47,13 @@ module.exports = function dispatchRequest(config) {
     }
   );
 
+  // Remove header where value is null
+  utils.forEach(config.headers, function deleteNullValueHeaders(value, key) {
+    if (value === null) {
+      delete config.headers[key];
+    }
+  });
+
   var adapter = config.adapter || defaults.adapter;
 
   return adapter(config).then(function onAdapterResolution(response) {