api/client.go: implement Client.Subscribe correctly by returning a error on unsupported channel type.

api/client_test.go

@@ -0,0 +1,26 @@
+package api
+
+import (
+	"testing"
+)
+
+func TestClient_Subscribe(t *testing.T) {
+	tests := []struct {
+		name    string
+		channel Channel
+		wantErr bool
+	}{
+		{"Channel", Channel{}, true},
+		{"ActionChannel", ActionChannel{}.Channel(), false},
+		{"HeartbeatChannel", HeartbeatChannel, false},
+		{"TransactionChannel", TransactionChannel, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := Client{}
+			if err := c.Subscribe(tt.channel); (err != nil) != tt.wantErr {
+				t.Errorf("Client.Subscribe() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}