First Should Return DefaultPageSize (graphql-dotnet#677)

* First Should Return DefaultPageSize

If the `first` and `last` properties are not set, `first` should default to the `defaultPageSize`.

* Update ResolveConnectionContext.cs

Author: RehanSaeed

src/GraphQL/Builders/ResolveConnectionContext.cs

@@ -17,6 +17,20 @@ public ResolveConnectionContext(ResolveFieldContext context, bool isUnidirection
         public bool IsUnidirectional { get; private set; }
 
         public int? First
+        {
+            get
+            {
+                var first = FirstInternal;
+                if (!first.HasValue && !Last.HasValue)
+                {
+                    return _defaultPageSize;
+                }
+                
+                return first;
+            }
+        }
+        
+        private int? FirstInternal
         {
             get
             {