Unwrap non-null types during selection type inference

We ran into the following error when we had a non-null `edges` field in
our schema (with a non-null type within the collection):

    Cannot read property 'cursor' of undefined

This patch adds a similar non-null `edges` field to the schema and adds
code to unwrap non-null types when inferring the type of a selection.

Author: jgwhite

addon/fields/selections/index.js

@@ -2,9 +2,22 @@ import createFieldInfo from '../info/create';
 import { getFieldNameAndAlias } from '../name';
 import { fragmentFieldsReducer, inlineFragmentFieldsReducer } from './fragments';
 import { partial } from '../../utils';
+import { GraphQLNonNull } from 'graphql';
 
-const getFieldType = (type, fieldName) =>
-  fieldName !== '__typename' && type._fields[fieldName].type;
+const getFieldType = (type, fieldName) => {
+  if (fieldName === '__typename') {
+    return null;
+  }
+
+  type = unwrapNonNull(type);
+
+  let field = type._fields[fieldName];
+  let fieldType = unwrapNonNull(field.type);
+
+  return fieldType;
+}
+
+const unwrapNonNull = (type) => type instanceof GraphQLNonNull ? type.ofType : type;
 
 export const selectedFieldsReducer =
   (getFieldNameAndAlias, getFieldType, createFieldInfo, getType, type, fragments, selections, selection) => {

tests/dummy/app/gql/schema.graphql

@@ -19,7 +19,7 @@ type LineItem {
 }
 
 type LineItemConnection {
-  edges: [LineItemEdge]
+  edges: [LineItemEdge!]!
   pageInfo: PageInfo!
 }