Merge pull request #610 from AArnott/fix544

Update System.Threading.Tasks.Extensions dependency to 4.5.3

Author: AArnott

src/MessagePack/MessagePack.csproj

@@ -36,7 +36,7 @@
   <ItemGroup>
     <PackageReference Include="System.Reflection.Emit" Version="4.3.0" />
     <PackageReference Include="System.Reflection.Emit.Lightweight" Version="4.3.0" />
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.3" />
     <PackageReference Include="System.Runtime.Serialization.Primitives" Version="4.3.0" />
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
   </ItemGroup>