Port upstream fix for z_importviewingkey bug

https://github.com/zcash/zcash/issues/2937

qa/rpc-tests/wallet_nullifiers.py

@@ -182,7 +182,7 @@ class WalletNullifiersTest (BitcoinTestFramework):
         # add node 1 address and node 2 viewing key to node 3
         myzvkey = self.nodes[2].z_exportviewingkey(myzaddr)
         self.nodes[3].importaddress(mytaddr1)
-        self.nodes[3].z_importviewingkey(myzvkey)
+        self.nodes[3].z_importviewingkey(myzvkey, 'whenkeyisnew', 1)
 
         # Check the address has been imported
         assert_equal(myzaddr in self.nodes[3].z_listaddresses(), False)

src/wallet/rpcdump.cpp

@@ -654,7 +654,7 @@ UniValue z_importviewingkey(const UniValue& params, bool fHelp)
     if (!EnsureWalletIsAvailable(fHelp))
         return NullUniValue;
 
-    if (fHelp || params.size() < 1 || params.size() > 2)
+    if (fHelp || params.size() < 1 || params.size() > 3)
         throw runtime_error(
             "z_importviewingkey \"vkey\" ( rescan startHeight )\n"
             "\nAdds a viewing key (as returned by z_exportviewingkey) to your wallet.\n"