Add initial action to repository config

- Now RepositoryState can have initial action
  if it is an initial state.
- Implement all needed tests.
- Fixes #281

spring-statemachine-data/mongodb/src/test/resources/data14.json

@@ -0,0 +1,27 @@
+[
+	{
+		"@id": "1",
+		"_class": "org.springframework.statemachine.data.mongodb.MongoDbRepositoryAction",
+		"spel": "extendedState.variables.put('foo', 0)"
+	},
+	{
+		"@id": "2",
+		"_class": "org.springframework.statemachine.data.mongodb.MongoDbRepositoryState",
+		"initial": true,
+		"initialAction" : "1",
+		"state": "S1"
+	},
+	{
+		"@id": "3",
+		"_class": "org.springframework.statemachine.data.mongodb.MongoDbRepositoryState",
+		"initial": false,
+		"state": "S2"
+	},
+	{
+		"_class": "org.springframework.statemachine.data.mongodb.MongoDbRepositoryTransition",
+		"source": "2",
+		"target": "3",
+		"event": "E1",
+		"kind": "EXTERNAL"
+	}
+]