[Types] Type of header in AxiosRequestConfig / for Axios.create is incorrect

[ghost91-]

Describe the bug

This is a follow up to #4184, as the problem still exists in version 1.0.0-alpha.1.

More concretely, The type of the headers property in Axios.create config doesn't support headers for specific requests method (or common).

To Reproduce

const axiosRequestConfig: AxiosRequestConfig = {
    headers: {
        get: { 'Accept-Language': 'en' } // same with common, or other http methods
    }
};

Axios.create(axiosRequestConfig);

results in the error

Type '{ get: { 'Accept-Language': string; }; }' is not assignable to type 'Partial<AxiosHeaders & MethodsHeaders & CommonHeaders>'.
  Property 'get' is incompatible with index signature.
    Type '{ 'Accept-Language': string; }' is not assignable to type 'string | number | boolean | string[] | undefined'.
      Object literal may only specify known properties, and ''Accept-Language'' does not exist in type 'string[]'.ts(2322)

Expected behavior

The above code example type checks successfully.

Environment

• Axios Version 1.0.0-alpha.1

Additional context/Screenshots

Related issues: #4184, #4193

[WillianAgostini]

Open PR #4927