Fix(crud) include param lookup now works w/plurals

Previously, {one,many}-to-many relationships with models would result in
`associationNames` that were plural. e.g. `Team` might have many
players and one location. The validation was expecting to see the plural
`Players` and the singular `Location` but Sequelize is expecting the
singular `Player` (`Location` worked fine). This meant that include
lookups would silently fail. This fixes the problem in a backward-
compatible way.

It continues to allow `include=Location` (capitalized) for backward-
compatibility. And now allows and actually does the lookup for
`include=players`, `include=player`, `include=Player`, `include=Players`
lookup relationships.

src/utils.js

@@ -20,6 +20,13 @@ export const parseInclude = request => {
   const { models } = noGetDb ? request : request.getDb();
 
   return include.map(a => {
+    const singluarOrPluralMatch = Object.keys(models).find((modelName) => {
+      const { _singular, _plural } = models[modelName];
+      return _singular === a || _plural === a;
+    });
+
+    if (singluarOrPluralMatch) return models[singluarOrPluralMatch];
+
     if (typeof a === 'string') return models[a];
 
     if (a && typeof a.model === 'string' && a.model.length) {