[fixes #1389] Problems with Through Associations with custom columnNames

[pberba]

I think this fixes issue #1389 (same as #1375 #1388) , when dealing with through associations, waterlines' many-to-many fails when custom columnNames of the join table is defined.

\node_modules\sails\node_modules\waterline\lib\waterline\model\lib\associationMethods\add.js:293
  var associationKey = collectionAttributes.attributes[attribute.on].via;
                                                                    ^

TypeError: Cannot read property 'via' of undefined

There seems to be a mix up with attribute.on and attribute.via, which is only different when the default columnNames are not used in a through association.

[sailsbot]

Hi @pepeberba! It looks like your pull request title doesn’t quite conform to our guidelines. Please edit the title so that it starts with [proposal], [patch], [fixes #], or [implements #]. Once you've fixed it, post a comment below (e.g. "ok, fixed!") and we'll take a look!

[pberba]

ok, fixed!