Fix behavior ObjectCollection for single item contains

src/libraries/System.Net.Http/src/System/Net/Http/Headers/ObjectCollection.cs

@@ -94,8 +94,9 @@ public void Clear()
         }
 
         public bool Contains(T item) =>
-            ReferenceEquals(item, _items) ||
-            (_size != 0 && _items is T[] items && Array.IndexOf(items, item, 0, _size) != -1);
+            _size == 0 || _items is null ? false :
+            _items is T o ? o.Equals(item) :
+            _items is T[] items && Array.IndexOf(items, item, 0, _size) != -1;
 
         public void CopyTo(T[] array, int arrayIndex)
         {
@@ -120,7 +121,7 @@ public void CopyTo(T[] array, int arrayIndex)
 
         public bool Remove(T item)
         {
-            if (ReferenceEquals(_items, item))
+            if (_items is T o && o.Equals(item))
             {
                 _items = null;
                 _size = 0;

src/libraries/System.Net.Http/tests/UnitTests/Headers/ObjectCollectionTest.cs

@@ -36,9 +36,29 @@ public void Ctor_ExecuteBothOverloads_MatchExpectation()
             c.Add("value1");
 
             Assert.Throws<InvalidOperationException>(() => { c.Add(null); });
+        }
+
+        [Fact]
+        public void ContainsAndRemove_UsesEqualitySemantics()
+        {
+            // Use default validator
+            ObjectCollection<string> c = new ObjectCollection<string>();
+
+            Assert.Equal(0, c.Count);
+            Assert.False(c.Contains("value" + 1));
+
+            c.Add("value" + 1);
 
             Assert.Equal(1, c.Count);
-            Assert.True(c.Contains("value1"));
+            // Force the reference to be different to ensure we are checking for semantic equality
+            // and not reference equality.
+            Assert.True(c.Contains("value" + 1));
+            c.Add("value" + 2);
+
+            Assert.True(c.Remove("value" + 1));
+            Assert.Equal(1, c.Count);
+
+            Assert.True(c.Contains("value" + 2));
         }
     }
 }