Add name to ArgumentNullExceptions (graphql-dotnet#752)

Author: SimonCropp

src/GraphQL/Resolvers/AsyncFieldResolver.cs

@@ -30,7 +30,7 @@ public class AsyncFieldResolver<TSourceType, TReturnType> : IFieldResolver<Task<
 
         public AsyncFieldResolver(Func<ResolveFieldContext<TSourceType>, Task<TReturnType>> resolver)
         {
-            _resolver = resolver ?? throw new ArgumentNullException("A resolver function must be specified");
+            _resolver = resolver ?? throw new ArgumentNullException(nameof(resolver), "A resolver function must be specified");
         }
 
         public Task<TReturnType> Resolve(ResolveFieldContext context)

src/GraphQL/Resolvers/FuncFieldResolver.cs

@@ -31,7 +31,7 @@ public FuncFieldResolver(Func<ResolveFieldContext<TSourceType>, TReturnType> res
         {
             if (resolver == null)
             {
-                throw new ArgumentNullException("A resolver function must be specified");
+                throw new ArgumentNullException(nameof(resolver), "A resolver function must be specified");
             }
             _resolver = resolver;
         }

src/GraphQL/StringExtensions.cs

@@ -49,7 +49,7 @@ public static string[] ToDelimitedArray(this string content, char delimiter)
         /// <param name="this">The this.</param>
         public static IEnumerable<char> ToEnumerable(this string @this)
         {
-            if (@this == null) throw new ArgumentNullException("@this");
+            if (@this == null) throw new ArgumentNullException(nameof(@this));
 
             for (var i = 0; i < @this.Length; ++i)
             {