Change headers type to string record

[remcohaszing]

This correctly reflects the type in a more type safe manner.

[remcohaszing]

@jasonsaayman Could you also have a look at this please? 😃

I’m also tagging @TimWolla because of their involvement in #2797

This forces TypeScript users to use objects whose values are strings as headers. I.e., currently the following is allowed:

import axios from 'axios';

axios.post('', {}, { headers: 'Invalid' })
axios.post('', {}, { headers: 13 })
axios.post('', {}, { headers: [] })
axios.post('', {}, { headers: { authorization: [] } })
// etc…

This changes the types, so only proper authorization headers are allowed, and the response header types are correct as well.

import axios from 'axios';

axios.post('', {}, { headers: { authorization: 'A string :)' } })

I don’t consider this to be a breaking change, as it still works the same for proper usage.

[TimWolla]

TIL about the Record type.

I don’t consider this to be a breaking change, as it still works the same for proper usage.

This is technically a breaking change, but only in a regard that it will find an actual bug in a consumer's code. TypeScript users will certainly appreciate the increased strictness in typing.