Fix expected timestamp value restJson1 defaults test

This commit updates the expected `defaultTimestamp` in a restJson1
defaults test to 2. The value 1 may have been incorrectly copied
from the corresponding test in Rails Json where the timestamp format
is httpdate instead of epoch seconds.

smithy-aws-protocol-tests/model/restJson1/defaults.smithy

@@ -323,7 +323,7 @@ apply OperationWithDefaults @httpResponseTests([
             defaultDocumentBoolean: false
             defaultDocumentList: ["b"]
             defaultNullDocument: "notNull"
-            defaultTimestamp: 1
+            defaultTimestamp: 2
             defaultBlob: "hi"
             defaultByte: 2
             defaultShort: 2
@@ -457,4 +457,4 @@ enum TestEnum {
 intEnum TestIntEnum {
     ONE = 1
     TWO = 2
-}
+}