indexOf body only if body is defined. Fixes #64.

[neojski]

I updated the PR. I call the callback now early so that we don't override the error with our No body found in response in case there was an error because most like it's going to be more informative.

[Pomax]

In that case I'd rather keep one callback, and then assign the error as

if(!body) {
  error = error || "No body found in response";
}

So we don't clobber the preexisting error, and we keep the code path a single path.

[neojski]

Done.

[Pomax]

:+1: