Fix API browser module emitting two content-type headers

Modules/Pages/Extensions.cs

@@ -7,7 +7,7 @@ namespace GenHTTP.Modules.Pages;
 
 public static class Extensions
 {
-    private static readonly ByteString HtmlContentType = new("text/html; charset=\"utf-8\"");
+    private static readonly ContentType HtmlContentType = new("text/html; charset=\"utf-8\"");
 
     /// <summary>
     /// Creates a response that can be returned by a handler to serve
@@ -17,8 +17,7 @@ public static class Extensions
     /// <param name="content">The HTML page to be served</param>
     /// <returns>The HTML page response</returns>
     public static IResponseBuilder GetPage(this IRequest request, string content) => request.Respond()
-                                                                                            .Content(new StringContent(content))
-                                                                                            .Header(KnownHeaders.ContentType, HtmlContentType);
+                                                                                            .Content(new StringContent(content, HtmlContentType));
 
     /// <summary>
     /// Escapes the given string so it can safely be used in HTML.

Testing/Acceptance/Modules/ApiBrowsing/ApplicationTests.cs

@@ -62,6 +62,8 @@ public async Task TestScalar(ServerEngine engine)
 
         await response.AssertStatusAsync(HttpStatusCode.OK);
 
+        Assert.AreEqual("text/html", response.Content.Headers.ContentType?.MediaType);
+
         AssertX.Contains("Scalar", await response.GetContentAsync());
     }