fix: proper fetch-spec related input handling

[AuHau]

Closes #83

This PR improves spec compliance of this library with accepting Request instance as input and several other small improvements (see comments).

Also, I am not sure how form-data package works with axios as axios should not accept anything else than DOM instance of form-data as you can see here, but it indeed works which leaves me bit buffled. But I would advise against using `form-data as it is non-compliant with the spec.

Btw. also I would recommend separating the compiled JS files into build directory as for example if you build the project and then run tests it uses the builder sources and not the TypeScript one.

[AuHau]

@jagoda @mdlavin could you please have a look on the PR? Thanks!

[AuHau]

@mdlavin I have fixed the extra option as you requested, but the JSON one is actually not working...