Fixed & Imporoved AxiosHeaders class (#5224)

* Refactored AxiosHeaders class;

* Added support for instances of AxiosHeaders as a value for the headers option;

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

lib/utils.js

@@ -228,7 +228,7 @@ const trim = (str) => str.trim ?
  * @param {Function} fn The callback to invoke for each item
  *
  * @param {Boolean} [allOwnKeys = false]
- * @returns {void}
+ * @returns {any}
  */
 function forEach(obj, fn, {allOwnKeys = false} = {}) {
   // Don't bother if no value provided
@@ -263,6 +263,24 @@ function forEach(obj, fn, {allOwnKeys = false} = {}) {
   }
 }
 
+function findKey(obj, key) {
+  key = key.toLowerCase();
+  const keys = Object.keys(obj);
+  let i = keys.length;
+  let _key;
+  while (i-- > 0) {
+    _key = keys[i];
+    if (key === _key.toLowerCase()) {
+      return _key;
+    }
+  }
+  return null;
+}
+
+const _global = typeof self === "undefined" ? typeof global === "undefined" ? this : global : self;
+
+const isContextDefined = (context) => !isUndefined(context) && context !== _global;
+
 /**
  * Accepts varargs expecting each argument to be an object, then
  * immutably merges the properties of each object and returns result.
@@ -282,16 +300,18 @@ function forEach(obj, fn, {allOwnKeys = false} = {}) {
  * @returns {Object} Result of all merge properties
  */
 function merge(/* obj1, obj2, obj3, ... */) {
+  const {caseless} = isContextDefined(this) && this || {};
   const result = {};
   const assignValue = (val, key) => {
-    if (isPlainObject(result[key]) && isPlainObject(val)) {
-      result[key] = merge(result[key], val);
+    const targetKey = caseless && findKey(result, key) || key;
+    if (isPlainObject(result[targetKey]) && isPlainObject(val)) {
+      result[targetKey] = merge(result[targetKey], val);
     } else if (isPlainObject(val)) {
-      result[key] = merge({}, val);
+      result[targetKey] = merge({}, val);
     } else if (isArray(val)) {
-      result[key] = val.slice();
+      result[targetKey] = val.slice();
     } else {
-      result[key] = val;
+      result[targetKey] = val;
     }
   }
 
@@ -527,6 +547,11 @@ const reduceDescriptors = (obj, reducer) => {
 
 const freezeMethods = (obj) => {
   reduceDescriptors(obj, (descriptor, name) => {
+    // skip restricted props in strict mode
+    if (isFunction(obj) && ['arguments', 'caller', 'callee'].indexOf(name) !== -1) {
+      return false;
+    }
+
     const value = obj[name];
 
     if (!isFunction(value)) return;
@@ -540,7 +565,7 @@ const freezeMethods = (obj) => {
 
     if (!descriptor.set) {
       descriptor.set = () => {
-        throw Error('Can not read-only method \'' + name + '\'');
+        throw Error('Can not rewrite read-only method \'' + name + '\'');
       };
     }
   });
@@ -609,5 +634,8 @@ export default {
   toObjectSet,
   toCamelCase,
   noop,
-  toFiniteNumber
+  toFiniteNumber,
+  findKey,
+  global: _global,
+  isContextDefined
 };