Defaults headers Content-Type don't allow "content-type" ?

[KiKiKi-KiKi]

Describe the issue

axios.defaults.headers.common['content-type'] = 'application/json';

This request header Content-Type become application/json, application/json;charset=utf-8.
GET / POST methods option can use 'content-type' too.
But defaults.headers is only allow the 'Content-Type' ?

Example Code

Global axios defaults

axios.defaults.headers.common['Content-Type'] = 'application/json';
// Request Header Content-Type: application/json

axios.defaults.headers.common['content-type'] = 'application/json';
// Request Header Content-Type: application/json, application/json;charset=utf-8

axios.defaults.headers.common['Content-type'] = 'application/json';
// Request Header Content-Type: application/json, application/json;charset=utf-8

Request methods config

axios.get(url, { headers: { 'Content-Type': 'application/json' } });
// Request Header Content-Type: application/json

axios.get(url, { headers: { 'content-type': 'application/json' } });
// Request Header Content-Type: application/json

axios.get(url, { headers: { 'Content-type': 'application/json' } });
// Request Header Content-Type: application/json

axios.get(url, { headers: { 'cOntEnt-tYpe': 'application/json' } });
// Request Header Content-Type: application/json

Content-Type application/json, application/json;charset=utf-8 requests ite become empty req.body in Express.

// Express API
const express = require('express');
const app = express();
app.use(express.json());

app.post('/api', (req, res) => {
  // Always `req.body` become `{}` 
  // When Content-Type is "application/json, application/json;charset=utf-8"
  req.body;
});

Expected behavior, if applicable

Request methods config can use Content-Type and content-type too. (This document was written the "content-type" in Response Schema )
But defaults.headers can use only Content-Type.
It's made me little confused. I sorry, I don't know if it's a specification or not.
if it's a specification, need a note of this on the "Global axios defaults" section, I think. Thank you.

Environment

• Axios Version [0.19.2]
• Adapter [XHR/HTTP]
• Browser [Chrome]
• Browser Version [83.0.4103.116]
• Node.js Version [v12.16.3]
• OS: [OSX 10.15.5]
• Additional Library Versions [React 16.13.1 / Express 4.17.1]

Additional context/Screenshots

[MoonSupport]

I think defaults.headers is only allow the 'Content-Type'.

For expected work, it has to call normalizeHeaderName function https://github.com/axios/axios/blob/405fe690f93264d591b7a64d006314e2222c8727/lib/helpers/normalizeHeaderName.js

But If you will directly change header like this

axios.defaults.headers.common['Content-Type'] = 'application/json';

There is nowhere to call this function.

But I agree your opinion need a note of this on the "Global axios defaults" section.

[karandeepvirk85]

I had this same issue couple of days ago. I had resolved this issue by using this header.

{ 'content-type': 'application/x-www-form-urlencoded' }
However I got data as POST in the API. I think it first encode URL with data to Axios module then Axios Post to API.

Here are screenshots
Sending Request

Browser

[MoonSupport]

@karandeepvirk85
Post api config call normalizeHeaderName.
Also Content-type Setting in Axios Module has a higher priority than user setting ( in post api or default )

[chinesedfan]

But defaults.headers is only allow the 'Content-Type' ?

See #2880.

This document was written the "content-type" in Response Schema

Note that it explains headers in response, which is different with axios config. And I'd like share more-stories-about-headers.

[MoonSupport]

Note that it explains headers in response, which is different with axios config. And I'd like share more-stories-about-headers.

After looking at your document, I think my explanation is wrong.

Now The headers setting is not case sensitive.
But regardless of the above the Content-Type cannot be set of it.

Is it right?