Check to make sure we're not already protected by ActiveSupport

ActiveSupport checks for YAML bug and throws an exception if the attribute for an XML element is set to "yaml". However, rack-post-body-to-params still thinks it is unsafe. This checks to see if the Hash::DisallowedType exception was raised and, if so, allows the initialize to happen still.

Without this fix (or one like it) then the middleware doesn't work, even when safe_yaml is present.