[5.3] Allow scopes starting with or boolean

[acasar]

Based on the feedback I am proposing a minor change in query scopes:

// Lets say we have the following scope:
public function scopeOrActive($query)
{
    $query->orWhere('active', 1);
}

// And we execute the following query:
User::where('foo', 'bar')->orActive();

Since in 5.2 scopes are automatically nested to avoid any confusion in logical order of the operators, we would get the following sql:

/* Note that the "or" boolean was replaced with "and" */
select * from "users" where ("foo"="bar") and ("active"="1")

However, it seems that some people would prefer the leading boolean to be respected:

select * from "users" where ("foo"="bar") or ("active"="1")

I am targeting this PR to 5.3, since it's a minor BC break.

[taylorotwell]

Can you send a PR to the 5.3 documentation (master branch) noting this change in the upgrade guide?