Update System.Net.Http to 4.3.2

src/Discord.Net.Rest/Discord.Net.Rest.csproj

@@ -9,6 +9,9 @@
   <ItemGroup>
     <ProjectReference Include="..\Discord.Net.Core\Discord.Net.Core.csproj" />
   </ItemGroup>
+  <ItemGroup Condition=" '$(TargetFramework)' != 'net45' ">
+    <PackageReference Include="System.Net.Http" Version="4.3.2" /> <!-- https://github.com/dotnet/corefx/issues/19535 -->
+  </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'net45' ">
     <Reference Include="System.Net.Http" />
   </ItemGroup>