Klarna::update does not clear data even if $clear is true

[cherudim]

Hi,

While reworking some code using this library we detected a problem where some invoices ended up with data from other orders as well, and after some investigation we found that the update-function in the Klarna class doesn't actually clear the object afterwards ($clear isn't used at all in that function), even thou the $clear-flag is set (by default) to true. Other similar functions like activate or reserveAmount does this, which makes me believe this is by omission.