Fix issue with Date as variable (graphql-dotnet#745)

fixes graphql-dotnet#744

Author: joemcbride

src/GraphQL.StarWars/GraphQL.StarWars.csproj

@@ -2,8 +2,7 @@
 
   <PropertyGroup>
     <Version>1.0.0</Version>
-    <TargetFrameworks>netstandard1.3;net45</TargetFrameworks>
-    <PackageTargetFallback>dnxcore50</PackageTargetFallback>
+    <TargetFrameworks>netstandard1.3;netstandard2.0;net45</TargetFrameworks>
     <AssemblyName>GraphQL.StarWars</AssemblyName>
     <PackageId>GraphQL.StarWars</PackageId>
     <PackageProjectUrl>https://github.com/graphql-dotnet/graphql-dotnet</PackageProjectUrl>

src/GraphQL.Tests/Types/DateGraphTypeTests.cs

@@ -10,6 +10,16 @@ public class DateGraphTypeTests
     {
         private readonly DateGraphType _type = new DateGraphType();
 
+        [Fact]
+        public void serialize_string_to_date()
+        {
+            CultureTestHelper.UseCultures(() =>
+            {
+                var actual = _type.Serialize("2018-07-24");
+                actual.ShouldBe("2018-07-24");
+            });
+        }
+
         [Fact]
         public void serialize_local_date_returns_date_only()
         {

src/GraphQL/Types/DateGraphType.cs

@@ -14,7 +14,9 @@ public DateGraphType()
 
         public override object Serialize(object value)
         {
-            if (value is DateTime dateTime)
+            var date = ParseValue(value);
+
+            if (date is DateTime dateTime)
             {
                 return dateTime.ToString("yyyy-MM-dd");
             }