Do not set host if it's not explicitely set in config (#6862)

Author: benjaminpetit

src/Libraries/Microsoft.Extensions.ServiceDiscovery.Yarp/ServiceDiscoveryDestinationResolver.cs

@@ -89,26 +89,13 @@ public async ValueTask<ResolvedDestinationCollection> ResolveDestinationsAsync(I
             }
 
             var name = $"{originalName}[{addressString}]";
-            string? resolvedHost;
+            string? resolvedHost = null;
 
             // Use the configured 'Host' value if it is provided.
             if (!string.IsNullOrEmpty(originalConfig.Host))
             {
                 resolvedHost = originalConfig.Host;
             }
-            else if (uri.IsLoopback)
-            {
-                // If there is no configured 'Host' value and the address resolves to localhost, do not set a host.
-                // This is to account for non-wildcard development certificate.
-                resolvedHost = null;
-            }
-            else
-            {
-                // Excerpt from RFC 9110 Section 7.2: The "Host" header field in a request provides the host and port information from the target URI [...]
-                // See: https://www.rfc-editor.org/rfc/rfc9110.html#field.host
-                // i.e, use Authority and not Host.
-                resolvedHost = originalUri.Authority;
-            }
 
             var config = originalConfig with { Host = resolvedHost, Address = resolvedAddress, Health = healthAddress };
             results.Add((name, config));

test/Libraries/Microsoft.Extensions.ServiceDiscovery.Yarp.Tests/YarpServiceDiscoveryTests.cs

@@ -188,8 +188,7 @@ public async Task ServiceDiscoveryDestinationResolverTests_Configuration_Host_Va
                 }
                 else
                 {
-                    // For non-localhost values, fallback to the input address.
-                    Assert.Equal("basket", a.Host);
+                    Assert.Null(a.Host);
                 }
             });
     }