Combine scope conditions using AND

Currently Post.active.inactive will result in Post.inactive since
the last where clause wins when scopes are merged.

This pull request will merge all scopes ( barring defaul scope)
using AND.

The default scope will be overridden if another scope acts on the
same where clause.

closes #7365
Fix warnings
Merge pull request #9553 from neerajdotname/7365-mergin-scopes-and-where

7365 merging scopes for where clauses
Write a more comprehensive CHANGELOG message [ci skip]