Implement RepeatedField.AddRange.

This fixes issue #1730.

csharp/src/Google.Protobuf.Test/Collections/RepeatedFieldTest.cs

@@ -74,6 +74,16 @@ namespace Google.Protobuf.Collections
             Assert.AreEqual("bar", list[1]);
         }
 
+        [Test]
+        public void AddRange()
+        {
+            var list = new RepeatedField<string>();
+            list.AddRange(new[] { "foo", "bar" });
+            Assert.AreEqual(2, list.Count);
+            Assert.AreEqual("foo", list[0]);
+            Assert.AreEqual("bar", list[1]);
+        }
+
         [Test]
         public void Add_RepeatedField()
         {

csharp/src/Google.Protobuf/Collections/RepeatedField.cs

@@ -307,7 +307,7 @@ namespace Google.Protobuf.Collections
         /// Adds all of the specified values into this collection.
         /// </summary>
         /// <param name="values">The values to add to this collection.</param>
-        public void Add(IEnumerable<T> values)
+        public void AddRange(IEnumerable<T> values)
         {
             ProtoPreconditions.CheckNotNull(values, nameof(values));
             // TODO: Check for ICollection and get the Count, to optimize?
@@ -317,6 +317,18 @@ namespace Google.Protobuf.Collections
             }
         }
 
+        /// <summary>
+        /// Adds all of the specified values into this collection. This method is present to
+        /// allow repeated fields to be constructed from queries within collection initializers.
+        /// Within non-collection-initializer code, consider using the equivalent <see cref="AddRange"/>
+        /// method instead for clarity.
+        /// </summary>
+        /// <param name="values">The values to add to this collection.</param>
+        public void Add(IEnumerable<T> values)
+        {
+            AddRange(values);
+        }
+
         /// <summary>
         /// Returns an enumerator that iterates through the collection.
         /// </summary>