Update source.go

[KiedaTamashi]

增加一个获取result， 为了获取[]byte

[GwynethLlewelyn]

I don't quite get it. What is the difference between the existing toResult() and your own proposed ToRawResult()? AFAICS, they're identical — except that ToRawResult() gets exported by the Tinify package, while toResult() does not.

I'd suggest one of the following instead...

1. Export toResult(), i.e. rename it as ToResult() (and of course search & replace that on the rest of the code)
2. Call toResult() from ToRawResult() instead of duplicating code, e.g.:

  func (s *Source) ToRawResult() (r *Result, err error) {
      return s.toResult()
  }

That way, if toResult() (internally) changes, ToRawResult() will continue to point to the right place; if additional processing is required by ToRawResult(), well, it can always be added to it — without interfering with whatever toResult() is actually returning.

Unless, of course, I'm not really understanding the point of this PR :)