fix: support nullable value type collection in queries (#1926)

Refit.Tests/RestService.cs

@@ -298,6 +298,9 @@ public interface IQueryApi
 
     [Get("/foo?{key}={value}")]
     Task ParameterMappedQuery(string key, string value);
+
+    [Get("/foo")]
+    Task NullableIntCollectionQuery([Query] int?[] values);
 }
 
 public interface IFragmentApi
@@ -2456,6 +2459,24 @@ public async Task ParameterMappedQueryShouldEscape()
         mockHttp.VerifyNoOutstandingExpectation();
     }
 
+    [Fact]
+    public async Task NullableIntCollectionQuery()
+    {
+        var mockHttp = new MockHttpMessageHandler();
+        var settings = new RefitSettings { HttpMessageHandlerFactory = () => mockHttp, };
+
+        mockHttp
+            .Expect(HttpMethod.Get, "https://github.com/foo")
+            .WithExactQueryString("values=3%2C4%2C")
+            .Respond(HttpStatusCode.OK);
+
+        var fixture = RestService.For<IQueryApi>("https://github.com", settings);
+
+        await fixture.NullableIntCollectionQuery([3, 4, null]);
+
+        mockHttp.VerifyNoOutstandingExpectation();
+    }
+
     [Fact]
     public async Task ShouldStripFragment()
     {

Refit/RequestBuilderImplementation.cs

@@ -1349,12 +1349,15 @@ static bool DoNotConvertToQueryMap(object? value)
             type = intType.GetGenericArguments()[0];
             return ShouldReturn(type);
 
+            // Check if type is a simple string or IFormattable type, check underlying type if Nullable<T>
             static bool ShouldReturn(Type type) =>
-                type == typeof(string)
-                || type == typeof(bool)
-                || type == typeof(char)
-                || typeof(IFormattable).IsAssignableFrom(type)
-                || type == typeof(Uri);
+                Nullable.GetUnderlyingType(type) is { } underlyingType
+                    ? ShouldReturn(underlyingType)
+                    : type == typeof(string)
+                      || type == typeof(bool)
+                      || type == typeof(char)
+                      || typeof(IFormattable).IsAssignableFrom(type)
+                      || type == typeof(Uri);
         }
 
         static void SetHeader(HttpRequestMessage request, string name, string? value)