Tests: add more Json and Xml tests

Author: 64J0

tests/Giraffe.Tests/Giraffe.Tests.fsproj

@@ -20,6 +20,8 @@
     <Compile Include="HttpContextExtensionsTests.fs" />
     <Compile Include="StreamingTests.fs" />
     <Compile Include="PreconditionalTests.fs" />
+    <Compile Include="JsonTests.fs" />
+    <Compile Include="XmlTests.fs" />
   </ItemGroup>
 
   <ItemGroup>