fix: loosing request header (#4858)

[WillianAgostini]

Fixes #4858

As described by #4858 there are problems that was loosing request headers.

Fix the error when pass headers parameters from class.

class Header {};
let myHeaders = new Header();
myHeaders["X-Foo"] = "bar";
myHeaders["X-Test"] = "test";
myHeaders["X-Time"] = Date.now();
  
axios.post('http://cors-test.appspot.com/test', { name: 'foo' }, { headers: myHeaders })
.then((res) => {
       console.log('res---',res.config.headers);
 })