error message for bad headers format is unclear

[aghArdeshir]

Describe the bug

I was writing bad code. in my GET request, for config I was giving a string to it.
it made me look a lot for problem and double-check my variables
The only error I was given was name.toUpperCase is not a function

To Reproduce

return axios.post(
      `/api/bind_client`,
      { panel_id: panelId, client_ids: [clientId] },
      {
        headers:  Authorization: authService.getAuthToken() 
      }
    );

This is wrong 👆
And this is the right format I should have used:

return axios.post(
      `/api/bind_client`,
      { panel_id: panelId, client_ids: [clientId] },
      {
        headers: { Authorization: authService.getAuthToken() }
      }
    );

Expected behavior

I expected it to just tell me headers cant be strings, or headers must be an object, etc...

Environment:

• Axios Version [e.g. 0.18.0] 0.18.0
• OS: [e.g. iOS 12.1.0, OSX 10.13.4] Windows 10
• Browser [e.g. Chrome, Safari] FireFox
• Browser Version [e.g. 22] 66.0.3 (64-bit)
• Additional Library Versions [e.g. React 16.7, React Native 0.58.0] React 16.8.6

Additional context/Screenshots

[jasonsaayman]

See #4209