Support axios `getUri`

[Leonardo Matos]

https://github.com/axios/axios#config-defaults

As "the goal is to support most/all features", supporting all instance methods also seems to be an important feature.

Currently, something like:

let uri = axios.getUri(axiosConfig)

Results in:

TypeError: axios.getUri is not a function

getUri method is specially useful when working with multiple instances, each one with a different baseURL.