Avoid body method parse errors on 204 responses

[sholladay]

Closes #193

This PR short-circuits the .json() body method and makes it return an empty string instead of parsing the body when the response status is 204 No Content. That way we avoid throwing a parse error due to an empty body.

Note that, as of this writing, we make no attempt to read the response body to check if it is empty, rather we trust the response status code from the server.

[sindresorhus]

Can you update index.d.ts too?

[sholladay]

Should be ready to merge.

Since we decided to go with empty string, the TypeScript types don't need to be updated.