Resolves Issue #831

Running the following code exercised the problem, and now works with
this change:

```
dynamodb.put_item(
  table_name: "foo",
  item: {
    id: 1,
    contents: StringIO.new("bar")
  }
)
```

The issue appeared to be that #read was called twice on the StringIO or
IO object, and the second call would fail as String#read does not
exist.

The hash and array logic used the format method changed in the previous
diff, so those worked correctly from the outset. Sets, however, had
special logic which required a second change.