Upgrade to Microsoft.CSharp v4.5.0

Author: mythz

NuGet/ServiceStack.Text/servicestack.text.nuspec

@@ -24,12 +24,12 @@
       </group>
       <group targetFramework="netstandard2.0">
         <dependency id="System.Runtime" version="[4.3.0, )" />
-        <dependency id="Microsoft.CSharp" version="[4.4.0, )" />
+        <dependency id="Microsoft.CSharp" version="[4.5.0, )" />
         <dependency id="System.Memory" version="[4.5.1, )" />
       </group>
       <group targetFramework="netcoreapp2.1">
         <dependency id="System.Runtime" version="[4.3.0, )" />
-        <dependency id="Microsoft.CSharp" version="[4.4.0, )" />
+        <dependency id="Microsoft.CSharp" version="[4.5.0, )" />
         <dependency id="System.Memory" version="[4.5.1, )" />
       </group>
     </dependencies>