Workaround for missing implementation of 'pending' tag (#2178)

The transaction spammer from Kurtosis requests the nonce value of its
account based on 'pending' tag. As we lack that implementation, go with
the next best answer from the 'latest' tag. This does not include info
about transactions from the mempool that have not yet been executed.

nimbus/rpc/rpc_utils.nim

@@ -46,7 +46,11 @@ proc headerFromTag*(chain: CoreDbRef, blockId: BlockTag): BlockHeader
     of "finalized": result = chain.finalizedHeader()
     of "pending":
       #TODO: Implement get pending block
-      raise newException(ValueError, "Pending tag not yet implemented")
+      # We currently fall back to `latest` so that the `tx-spammer` in
+      # `kurtosis-tech/ethereum-package` can make progress. A real
+      # implementation is still required that takes into account any
+      # pending transactions that have not yet been bundled into a block.
+      result = chain.getCanonicalHead()
     else:
       raise newException(ValueError, "Unsupported block tag " & tag)
   else: