Incorrect type ResponseOptions used in README docs

[nicolasff]

Hello,

Thanks for making this great library! It seems to work well, although in the process of trying it out I did find a minor issue with the docs.

The type for the options object was incorrectly documented as being ResponseOptions, when it's actually RequestOptions. I thought it may have been left over from a previous release, but looking through the commits this type doesn't seem to have ever existed).

I was confused for a little while today after adding Fetch to my project by following the README, and getting a compilation error:

src/main.cpp:56:3: error: 'ResponseOptions' was not declared in this scope
   ResponseOptions options;
   ^

In retrospect it makes much more sense for it to be an instance of RequestOptions when I'm sending a request, not configuring the response – I'm not sure what this would even mean :-)