Added all content headers

Author: alexeyzimarev

src/RestSharp/KnownHeaders.cs

@@ -13,11 +13,21 @@
 // limitations under the License.
 // 
 
+// ReSharper disable InconsistentNaming
+
 namespace RestSharp;
 
 public static class KnownHeaders {
     public const string Accept             = "Accept";
+    public const string Allow              = "Allow";
+    public const string Expires            = "Expires";
+    public const string ContentDisposition = "Content-Disposition";
     public const string ContentEncoding    = "Content-Encoding";
+    public const string ContentLanguage    = "Content-Language";
+    public const string ContentLength      = "Content-Length";
+    public const string ContentLocation    = "Content-Location";
+    public const string ContentRange       = "Content-Range";
     public const string ContentType        = "Content-Type";
-    public const string ContentDisposition = "Content-Disposition";
+    public const string LastModified       = "Last-Modified";
+    public const string ContentMD5         = "Content-MD5";
 }

src/RestSharp/Request/RequestContent.cs

@@ -24,7 +24,11 @@ class RequestContent : IDisposable {
     readonly RestRequest  _request;
     readonly List<Stream> _streams = new();
 
-    internal static readonly string[] ContentHeaders = { ContentType, ContentEncoding, ContentDisposition };
+    internal static readonly string[] ContentHeaders = {
+        Allow, Expires,
+        ContentDisposition, ContentEncoding, ContentLanguage, ContentLength, ContentLocation, ContentRange, ContentType, ContentMD5,
+        LastModified
+    };
 
     HttpContent? Content { get; set; }
 
@@ -147,6 +151,7 @@ void AddHeaders() {
             // We need some content to add content headers to it, so if necessary, we'll add empty content
             Content = new StringContent("");
         }
+
         contentHeaders.ForEach(AddHeader);
 
         void AddHeader(Parameter parameter) {

src/RestSharp/Request/RequestParameters.cs

@@ -53,9 +53,9 @@ public RequestParameters AddDefaultParameters(RestClient client) {
 
     // Add Accept header based on registered deserializers if none has been set by the caller.
     public RequestParameters AddAcceptHeader(RestClient client) {
-        if (_requestParameters.All(p => !p.Name!.EqualsIgnoreCase("accept"))) {
+        if (_requestParameters.All(p => !p.Name!.EqualsIgnoreCase(KnownHeaders.Accept))) {
             var accepts = string.Join(", ", client.AcceptedContentTypes);
-            _requestParameters.Add(new Parameter("Accept", accepts, ParameterType.HttpHeader));
+            _requestParameters.Add(new Parameter(KnownHeaders.Accept, accepts, ParameterType.HttpHeader));
         }
 
         return this;