Add hashCode implementations where there are none

src/main/java/com/fasterxml/jackson/databind/node/BooleanNode.java

@@ -77,6 +77,11 @@ public final void serialize(JsonGenerator jg, SerializerProvider provider)
         jg.writeBoolean(_value);
     }
 
+    @Override
+    public int hashCode() {
+        return Boolean.valueOf(_value).hashCode();
+    }
+
     @Override
     public boolean equals(Object o)
     {

src/main/java/com/fasterxml/jackson/databind/node/IntNode.java

@@ -129,5 +129,5 @@ public boolean equals(Object o)
     }
 
     @Override
-        public int hashCode() { return _value; }
+    public int hashCode() { return _value; }
 }

src/main/java/com/fasterxml/jackson/databind/node/MissingNode.java

@@ -95,4 +95,9 @@ public String toString()
         // toString() should never return null
         return "";
     }
+
+    @Override
+    public int hashCode() {
+        return 0;
+    }
 }

src/main/java/com/fasterxml/jackson/databind/node/NullNode.java

@@ -53,4 +53,9 @@ public boolean equals(Object o)
     {
         return (o == this);
     }
+
+    @Override
+    public int hashCode() {
+        return 0;
+    }
 }

src/main/java/com/fasterxml/jackson/databind/node/ValueNode.java

@@ -45,6 +45,8 @@ public void serializeWithType(JsonGenerator jg, SerializerProvider provider,
         typeSer.writeTypeSuffixForScalar(this, jg);
     }
 
+    @Override public abstract int hashCode();
+
     /*
     /**********************************************************************
     /* Base impls for standard methods