Wrapping up the PR

Author: alexeyzimarev

src/RestSharp/KnownHeaders.cs

@@ -14,6 +14,7 @@
 // 
 
 // ReSharper disable InconsistentNaming
+// ReSharper disable MemberCanBePrivate.Global
 
 namespace RestSharp;
 
@@ -32,4 +33,13 @@ public static class KnownHeaders {
     public const string LastModified       = "Last-Modified";
     public const string ContentMD5         = "Content-MD5";
     public const string Host               = "Host";
-}
+
+    internal static readonly string[] ContentHeaders = {
+        Allow, Expires, ContentDisposition, ContentEncoding, ContentLanguage, ContentLength, ContentLocation, ContentRange, ContentType, ContentMD5,
+        LastModified
+    };
+
+    static readonly HashSet<string> ContentHeadersHash = new(ContentHeaders.Select(x => x.ToLower()));
+
+    internal static bool IsContentHeader(string key) => ContentHeadersHash.Contains(key.ToLower());
+}

src/RestSharp/Request/HttpRequestMessageExtensions.cs

@@ -20,7 +20,7 @@ namespace RestSharp;
 
 static class HttpRequestMessageExtensions {
     public static void AddHeaders(this HttpRequestMessage message, RequestHeaders headers) {
-        var headerParameters = headers.Parameters.Where(x => !RequestContent.ContentHeaders.Contains(x.Name));
+        var headerParameters = headers.Parameters.Where(x => !KnownHeaders.IsContentHeader(x.Name!));
 
         headerParameters.ForEach(x => AddHeader(x, message.Headers));
 
@@ -31,4 +31,4 @@ void AddHeader(Parameter parameter, HttpHeaders httpHeaders) {
             httpHeaders.TryAddWithoutValidation(parameter.Name!, parameterStringValue);
         }
     }
-}
+}

src/RestSharp/Request/RequestContent.cs

@@ -26,19 +26,7 @@ class RequestContent : IDisposable {
     readonly RestRequest  _request;
     readonly List<Stream> _streams = new();
 
-    internal static readonly string[] ContentHeaders = {
-        Allow.ToLower(),
-        Expires.ToLower(),
-        ContentDisposition.ToLower(),
-        ContentEncoding.ToLower(),
-        ContentLanguage.ToLower(),
-        ContentLength.ToLower(),
-        ContentLocation.ToLower(),
-        ContentRange.ToLower(),
-        ContentType.ToLower(),
-        ContentMD5.ToLower(),
-        LastModified.ToLower(),
-    };
+    
 
     HttpContent? Content { get; set; }
 
@@ -172,15 +160,15 @@ void AddPostParameters(ParametersCollection? postParameters) {
             var formContent = new FormUrlEncodedContent(
                 _request.Parameters
                     .Where(x => x.Type == ParameterType.GetOrPost)
-                    .Select(x => new KeyValuePair<string, string>(x.Name!, x.Value!.ToString()!))!
+                    .Select(x => new KeyValuePair<string, string>(x.Name!, x.Value!.ToString()!))
             );
             Content = formContent;
         }
     }
 
     void AddHeaders() {
         var contentHeaders = _request.Parameters
-            .Where(x => x.Type == ParameterType.HttpHeader && ContentHeaders.Contains(x.Name?.ToLower()))
+            .Where(x => x.Type == ParameterType.HttpHeader && IsContentHeader(x.Name!))
             .ToArray();
 
         if (contentHeaders.Length > 0 && Content == null) {

src/RestSharp/Request/RequestHeaders.cs

@@ -13,6 +13,7 @@
 // limitations under the License.
 // 
 
+// ReSharper disable InvertIf
 namespace RestSharp; 
 
 class RequestHeaders {

test/RestSharp.Tests/RestContentTests.cs

@@ -1,18 +1,15 @@
-using System.Net;
-
-namespace RestSharp.Tests;
+namespace RestSharp.Tests;
 
 public class RestContentTests {
     [Fact]
     public void RestContent_CaseInsensitiveHeaders() {
-        var myContentType = "application/x-custom";
-        var request = new RestRequest("resource");
-        request.AddHeader("coNteNt-TypE", myContentType);
-        var client = new RestClient();
-        var content = new RequestContent(client, request);
+        const string myContentType = "application/x-custom";
+
+        var request = new RestRequest("resource").AddHeader("coNteNt-TypE", myContentType);
+        var content = new RequestContent(new RestClient(), request);
 
         var httpContent = content.BuildContent();
 
-        Assert.Equal(myContentType, httpContent.Headers.ContentType.MediaType);
+        httpContent.Headers.ContentType!.MediaType.Should().Be(myContentType);
     }
 }