Fix IO Handling in DynamoDB Sets

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.

aws-sdk-core/lib/aws-sdk-core/dynamodb/attribute_value.rb

@@ -51,7 +51,7 @@ def format_set(set)
           case set.first
           when String then { ss: set.map(&:to_s) }
           when Numeric then { ns: set.map(&:to_s) }
-          when StringIO, IO then { bs: set.map(&:read) }
+          when StringIO, IO then { bs: set.to_a }
           else
             msg = "set types only support String, Numeric, or IO objects"
             raise ArgumentError, msg

aws-sdk-core/spec/aws/dynamodb/attribute_value_spec.rb

@@ -26,7 +26,7 @@ module DynamoDB
           ])
         end
 
-        it 'converts IO objects to :b (blob)' do
+        it 'converts IO objects to :b (binary)' do
           io = StringIO.new('bar')
           formatted = value.marshal(foo: io)
           expect(formatted[:m]["foo"][:b].read).to eq('bar')
@@ -43,8 +43,9 @@ module DynamoDB
         end
 
         it 'converts binary sets to :bs (binary set)' do
-          formatted = value.marshal(Set.new([StringIO.new('data')]))
-          expect(formatted).to eq(bs: ['data'])
+          io_obj = StringIO.new('data')
+          formatted = value.marshal(Set.new([io_obj]))
+          expect(formatted).to eq(bs: [io_obj])
         end
 
         it 'converts numerics to :n (number)' do