Add MS.Extensions.Primitives to .NET Core nupkg

Author: xplicit

NuGet.Core/ServiceStack.Text.Core/servicestack.text.core.nuspec

@@ -20,6 +20,7 @@
     <copyright>ServiceStack 2017 and contributors</copyright>
     <dependencies>
       <group targetFramework=".NETStandard1.1">
+        <dependency id="Microsoft.Extensions.Primitives" version="[1.1.1, )" />
         <dependency id="System.Runtime.Serialization.Primitives" version="[4.3.0, )" />
         <dependency id="System.Runtime.Serialization.Xml" version="[4.3.0, )" />
         <dependency id="System.Net.Requests" version="[4.3.0, )" />
@@ -32,6 +33,7 @@
         <dependency id="System.Runtime.InteropServices.RuntimeInformation" version="[4.3.0, )" />
       </group>
       <group targetFramework=".NETStandard1.3">
+        <dependency id="Microsoft.Extensions.Primitives" version="[1.1.1, )" />
         <dependency id="System.Runtime.Serialization.Primitives" version="[4.3.0, )" />
         <dependency id="System.Runtime.Serialization.Xml" version="[4.3.0, )" />
         <dependency id="System.Net.Requests" version="[4.3.0, )" />