Replace IsDisposable with Dispose + Refactor

[mariusz96]

As noted in #19 (comment), IsDisposable gets replaced with Dispose:

public class MiddlewareResolverResult
{
-    /// <summary>
-    /// Gets or sets the value indicating whether the middleware should be disposed.
-    /// Set this to <see langword="true"/> if the middleware is IDisposable or
-    /// IAsyncDisposable (requires .NET Standard 2.1 or greater)
-    /// and is not disposed by a dependency injection container.
-    /// </summary>
-    public bool IsDisposable { get; set; }

+    /// <summary>
+    /// Gets or sets the value indicating whether the middleware should be disposed.
+    /// Set this to <see langword="true"/> if the middleware is not disposed
+    /// by a dependency injection container.
+    /// </summary>
+    public bool Dispose { get; set; }
}

and it is no longer needed to check if middleware implements IDisposable/IAsyncDisposable in IMiddlewareResolver implementations.

And I refactored stuff for readability and less code duplication.