Enforce immutability of IterableEnvelope through its iterator

Ref: #862

src/main/java/org/cactoos/iterable/IterableEnvelope.java

@@ -25,6 +25,7 @@
 
 import java.util.Iterator;
 import org.cactoos.Scalar;
+import org.cactoos.iterator.Immutable;
 import org.cactoos.scalar.UncheckedScalar;
 
 /**
@@ -54,7 +55,9 @@ public IterableEnvelope(final Scalar<Iterable<X>> scalar) {
 
     @Override
     public final Iterator<X> iterator() {
-        return this.iterable.value().iterator();
+        return new Immutable<>(
+            this.iterable.value().iterator()
+        );
     }
 
 }

src/test/java/org/cactoos/collection/CollectionEnvelopeTest.java

@@ -32,9 +32,6 @@
  * Test case for {@link CollectionEnvelope}.
  *
  * @since 0.32
- * @todo #813:30min IterableEnvelope appears to be read only but it returns
- *  mutable Iterator through which user can remove elements. Implement immutable
- *  Iterator for IterableEnvelope `iterator()` method.
  * @checkstyle JavadocMethodCheck (500 lines)
  */
 public final class CollectionEnvelopeTest {

src/test/java/org/cactoos/iterable/IterableEnvelopeTest.java

@@ -0,0 +1,52 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2017-2018 Yegor Bugayenko
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+package org.cactoos.iterable;
+
+import java.util.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+import org.junit.Test;
+
+/**
+ * Test case for {@link IterableEnvelope}.
+ *
+ * @since 0.35
+ * @checkstyle JavadocMethodCheck (500 lines)
+ */
+public final class IterableEnvelopeTest {
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void returnsIteratorWithUnsupportedRemove() {
+        final IterableEnvelope<String> list = new IterableEnvelope<String>(
+            () -> {
+                final List<String> inner = new LinkedList<>();
+                inner.add("eleven");
+                return inner;
+            }
+        ) { };
+        final Iterator<String> iterator = list.iterator();
+        iterator.next();
+        iterator.remove();
+    }
+}