problem: doesn't get head updates from all available heads

solution: ensure all heads are started on multihead start
rel: #67

src/test/groovy/io/emeraldpay/dshackle/upstream/MergedHeadSpec.groovy

@@ -0,0 +1,67 @@
+/**
+ * Copyright (c) 2021 EmeraldPay, Inc
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.emeraldpay.dshackle.upstream
+
+import org.springframework.context.Lifecycle
+import reactor.core.publisher.Flux
+import spock.lang.Specification
+
+class MergedHeadSpec extends Specification {
+
+    def "ensures that heads are running on start"() {
+        setup:
+        def head1 = Stub(TestHead1) {
+            _ * getFlux() >> Flux.empty()
+        }
+        def head2 = Mock(TestHead2) {
+            _ * isRunning() >> true
+            _ * getFlux() >> Flux.empty()
+        }
+        def head3 = Mock(TestHead2) {
+            _ * isRunning() >> false
+            _ * getFlux() >> Flux.empty()
+        }
+
+        when:
+        def merged = new MergedHead([head1, head2, head3])
+        merged.start()
+
+        then:
+        1 * head3.start()
+    }
+
+    class TestHead1 extends AbstractHead {
+
+    }
+
+    class TestHead2 extends AbstractHead implements Lifecycle {
+
+        @Override
+        void start() {
+
+        }
+
+        @Override
+        void stop() {
+
+        }
+
+        @Override
+        boolean isRunning() {
+            return false
+        }
+    }
+}