Don't Read IO Objects in Attribute Value Marshal

[awood45]

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.

One thing I'd like to review further before merging is the fact that maps and collections do this same #read call but do not have this problem when tested.

[trevorrowe]

The change looks good. Maps and lists definitely are effected, but I suspect what you meant to say is that they are not tested?

dynamodb.put_item(
  table_name: "foo",
  item: {
    id: 1,
    contents: { # map
      data: StringIO.new("bar")
    }
  }
)
#=> raises NoMethodError: undefined method `read' for "bar":String

dynamodb.put_item(
  table_name: "foo",
  item: {
    id: 1,
    contents: [ # list
      StringIO.new("bar"),
    ]
  }
)
#=> raises NoMethodError: undefined method `read' for "bar":String

[awood45]

Per our earlier discussion, Hashes and Arrays worked fine from the first change, the second use of #read was isolated to Set types only. This change fixes the appropriate test and code.