Update CHANGELOG for v1.5.13

***NO_CI***

Author: nfbot

CHANGELOG.md

@@ -2,7 +2,15 @@
 
 ## [**Changes available only in 'Preview' NuGet packages:**](https://github.com/nanoframework/System.Net.Http/tree/HEAD)
 
-[Full Changelog](https://github.com/nanoframework/System.Net.Http/compare/v1.5.8...HEAD)
+[Full Changelog](https://github.com/nanoframework/System.Net.Http/compare/v1.5.10...HEAD)
+
+**Fixed bugs:**
+
+- Fix HttpClient.Post error on subsequent call [\#339](https://github.com/nanoframework/System.Net.Http/pull/339)
+
+## [v1.5.10](https://github.com/nanoframework/System.Net.Http/tree/v1.5.10) (2022-08-04)
+
+[Full Changelog](https://github.com/nanoframework/System.Net.Http/compare/v1.5.8...v1.5.10)
 
 **Documentation and other chores:**