Exclude original_data field from PartialEq comparison

[Mr-Leshiy]

As original_data is not required to be set, used/initialised only in limited number of cases.
So to be still be consistent in comparison operation, I am suggesting to exclude original_data field from comparing implementation.
As a result it eliminates inconvenience to alway keep this field either empty or initialised to be able to properly compare two ProtectedHeaders instances at this moment.

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