title: Base64 encoding doesn't fail with invalid characters
body: <p>I am trying to ensure a string coming from an http request is valid for use in a base64 url param. I've been experimenting with <code>base64.RawURLEncoding</code> as I assumed encoding an invalid string would throw an err, or at least decoding the result of this would fail, however it quite happily encodes/decodes the string regardless of the input.</p>
  
  <p><a href="https://play.golang.org/p/3sHUfl2NSJK" rel="nofollow noreferrer">https://play.golang.org/p/3sHUfl2NSJK</a></p>
  
  <p>I have created the above playground showing the issue I'm having (albeit an extreme example). Is there another way of ascertaining whether a string consists entirely of valid base64 characters?</p>