Initialize Headers Dictionary Only Once (#4853)

Switch the  WebResponseObject.Headers Dictionary to initialize once instead of creating a new dictionary on every get. Moved logic to  WebResponseHelper.GetHeadersDictionary()  as this code will be reused outside of  WebResponseObject .

* [Feature] Create Headers Only Once and Only If Called

* {feature] headers -> _headers

* [Feature] Move logic to WebResponseObject for reuse

This code has potential reuse in InvokeRestMethodCommand. Moving it to WebResponseHelper

* Add comment to address PR Feedback.

src/Microsoft.PowerShell.Commands.Utility/commands/utility/WebCmdlet/CoreCLR/WebResponseHelper.CoreClr.cs

@@ -4,6 +4,8 @@
 Copyright (c) Microsoft Corporation.  All rights reserved.
 --********************************************************************/
 
+using System;
+using System.Collections.Generic;
 using System.Net.Http;
 using System.Globalization;
 
@@ -16,6 +18,28 @@ namespace Microsoft.PowerShell.Commands
             string characterSet = response.Content.Headers.ContentType.CharSet;
             return characterSet;
         }
+        
+        internal static Dictionary<string, IEnumerable<string>> GetHeadersDictionary(HttpResponseMessage response)
+        {
+            var headers = new Dictionary<string, IEnumerable<string>>(StringComparer.OrdinalIgnoreCase);
+            foreach (var entry in response.Headers)
+            {
+                headers[entry.Key] = entry.Value;
+            }
+            // In CoreFX, HttpResponseMessage separates content related headers, such as Content-Type to 
+            // HttpResponseMessage.Content.Headers. The remaining headers are in HttpResponseMessage.Headers.
+            // The keys in both should be unique with no duplicates between them.
+            // Added for backwards compatibility with PowerShell 5.1 and earlier.
+            if (response.Content != null)
+            {
+                foreach (var entry in response.Content.Headers)
+                {
+                    headers[entry.Key] = entry.Value;
+                }
+            }
+
+            return headers;
+        }
 
         internal static string GetProtocol(HttpResponseMessage response)
         {

src/Microsoft.PowerShell.Commands.Utility/commands/utility/WebCmdlet/CoreCLR/WebResponseObject.CoreClr.cs

@@ -31,23 +31,17 @@ namespace Microsoft.PowerShell.Commands
         {
             get
             {
-                var headers = new Dictionary<string, IEnumerable<string>>(StringComparer.OrdinalIgnoreCase);
-                foreach (var entry in BaseResponse.Headers)
+                if(_headers == null)
                 {
-                    headers[entry.Key] = entry.Value;
-                }
-                if (BaseResponse.Content != null)
-                {
-                    foreach (var entry in BaseResponse.Content.Headers)
-                    {
-                        headers[entry.Key] = entry.Value;
-                    }
+                    _headers = WebResponseHelper.GetHeadersDictionary(BaseResponse);
                 }
 
-                return headers;
+                return _headers;
             }
         }
 
+        private Dictionary<string, IEnumerable<string>> _headers = null;
+        
         /// <summary>
         /// gets the RelationLink property
         /// </summary>