Update ifdefs from DNXCORE50 to DOTNET5_4

Fixes #467

Author: pranavkm

Diff for: src/Microsoft.AspNet.Http/FormCollection.cs

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.Collections;
 using System.Collections.Generic;
 using Microsoft.Extensions.Primitives;
@@ -13,7 +14,7 @@ namespace Microsoft.AspNet.Http.Internal
     public class FormCollection : IFormCollection
     {
         public static readonly FormCollection Empty = new FormCollection();
-#if DNXCORE50
+#if DOTNET5_4
         private static readonly string[] EmptyKeys = Array.Empty<string>();
         private static readonly StringValues[] EmptyValues = Array.Empty<StringValues>();
 #else

Diff for: src/Microsoft.AspNet.Http/HeaderDictionary.cs

@@ -13,7 +13,7 @@ namespace Microsoft.AspNet.Http.Internal
     /// </summary>
     public class HeaderDictionary : IHeaderDictionary
     {
-#if DNXCORE50
+#if DOTNET5_4
         private static readonly string[] EmptyKeys = Array.Empty<string>();
         private static readonly StringValues[] EmptyValues = Array.Empty<StringValues>();
 #else

Diff for: src/Microsoft.AspNet.Http/QueryCollection.cs

@@ -14,7 +14,7 @@ namespace Microsoft.AspNet.Http.Internal
     public class QueryCollection : IQueryCollection
     {
         public static readonly QueryCollection Empty = new QueryCollection();
-#if DNXCORE50
+#if DOTNET5_4
         private static readonly string[] EmptyKeys = Array.Empty<string>();
         private static readonly StringValues[] EmptyValues = Array.Empty<StringValues>();
 #else

Diff for: src/Microsoft.AspNet.Http/RequestCookieCollection.cs

@@ -11,7 +11,7 @@ namespace Microsoft.AspNet.Http.Internal
     public class RequestCookieCollection : IRequestCookieCollection
     {
         public static readonly RequestCookieCollection Empty = new RequestCookieCollection();
-#if DNXCORE50
+#if DOTNET5_4
         private static readonly string[] EmptyKeys = Array.Empty<string>();
 #else
         private static readonly string[] EmptyKeys = new string[0];