Subtask of #6

When receipts for a celo transaction a re requested that is included in an block which has it's state pruned (currently after 128 blocks) we cannot access the basegasprice from the contract state any more.

In that case the fields of the receipt should be empty.

• Adjust core/types/receipt.go, Receipts.EncodeIndex to handle celo transactions as well
• Add tests for this and make sure common client libraries can handle this.