Atomic wrappers using Value incorrectly fail on CompareAndSwap

[prashantv]

While writing a fix + tests for #126, I realized where was a larger issue with how we're handling default values for wrapper-types. We have 2 representations for a default value: the underlying atomic Value is nil, or the atomic Value has the default value. However, having 2 representations causes issues for CompareAndSwap, since we only ever compare against the default value, so we'll never match the nil value.

This simple test shows the issue:

var s atomic.String
fmt.Println(s.CompareAndSwap("", "foo"))
// Prints: false. Expect true.

This can be worse if there's a loop using CompareAndSwap (typical way to use CompareAndSwap), since it will never succeed when the old value is read from Load(),

// s is an atomic.String.
// If s has never been set, the following loop will never end

for {
  old := s.Load()
  new := transform(old)
  if s.CompareAndSwap(old, new) {
    return nil
  }
}

The same above code will work for atomic.Int32 and other wrappers around int primitives. This is only an issue with Value primitives. From my testing, this affects String and Error, added in #111.

[prashantv]

Fixed in #130.