Fixing instance.defaults.headers type

[spalger]

When creating an Axios instance with Axios.create() the headers config (as described in the README) is merged into the root of instance.default.headers. These values are not accessible because the types for instance.defaults.headers do not include an index type to represent the arbitrary properties which are merged there.

This PR includes this index type in the instance.defaults.headers object and includes validation that this works in the test/typescript/axios.ts file. Without the change in index.d.ts this test shows the following error:

[jasonsaayman]

Thanks for the contribution 😄