Correct unmarshal for GetAddressBalanceCmd.

The unmarshaller for the GetAddressBalanceCmd was calling the wrong parse
function.  It wasn't caught by the tests because the error on
UnmarshalJSON was being ignored, so this commit also updates the tests to check the
error.

cmds.go

@@ -1636,7 +1636,7 @@ func (cmd *GetAddressBalanceCmd) UnmarshalJSON(b []byte) error {
 		return err
 	}
 
-	newCmd, err := parseListAllTransactionsCmd(&r)
+	newCmd, err := parseGetAddressBalanceCmd(&r)
 	if err != nil {
 		return err
 	}

cmds_test.go

@@ -346,7 +346,10 @@ func TestCmds(t *testing.T) {
 
 		// Read marshaled command back into c.  Should still
 		// match result.
-		c.UnmarshalJSON(mc)
+		if err := c.UnmarshalJSON(mc); err != nil {
+			t.Errorf("%s: error while unmarshalling: %v", test.name,
+				err)
+		}
 		if !reflect.DeepEqual(test.result, c) {
 			t.Errorf("%s: unmarshal not as expected. "+
 				"got %v wanted %v", test.name, spew.Sdump(c),