http: fix incomplete content read

HTTP/HttpStream.cs

@@ -75,11 +75,20 @@ class HttpStream : IDisposable
         }
     }
 
-    public ValueTask<int> Read(Memory<byte> buffer)
+    public async ValueTask<int> Read(Memory<byte> buffer)
     {
         try
         {
-            return Buffer.ReadAsync(buffer);
+            int index = 0;
+            while (index < buffer.Length)
+            {
+                int count = await Buffer.ReadAsync(buffer[index..]);
+                if (count == 0)
+                    break;
+
+                index += count;
+            }
+            return index;
         }
         catch (IOException e)
         {