fix polymorphic relation call exists? #15821.


After explore the differences for `JoinAssociation#join_constraints` between rails 4.2.0 alpha and 4.1.2.rc2, I find the method returns `JoinInformation` structure which carry a `bind_values` array and does not directly substitute the dynamic parameter holders `(?)` of sql statement anymore.

```ruby
# rails 4.1.2.rc2
# /active_record/associations/join_dependency/join_association.rb
def join_constraints(foreign_table, foreign_klass, node, join_type, tables, scope_chain, chain)
  #...
  if reflection.type
    constraint = constraint.and table[reflection.type].eq foreign_klass.base_class.name
  end

  joins
end

# Query
Post Load (0.1ms)  SELECT "posts".* FROM "posts" INNER JOIN "slugs" ON
 "slugs"."sluggable_id" = "posts"."id" AND "slugs"."sluggable_type" = 'Post' 
WHERE "slugs"."name" = 'the-slug'

# rails 4.2.0 alpha
# /active_record/associations/join_dependency/join_association.rb
def join_constraints(foreign_table, foreign_klass, node, join_type, tables, scope_chain, chain)
  #...
  if reflection.type
    # ...
    substitute = klass.connection.substitute_at(column, bind_values.length)
    bind_values.push [column, value]
    constraint = constraint.and table[reflection.type].eq substitute
  end
 
  JoinInformation.new joins, bind_values
end

# Query
Post Load (0.1ms)  SELECT "posts".* FROM "posts" INNER JOIN "slugs" ON 
"slugs"."sluggable_id" = "posts"."id" AND "slugs"."sluggable_type" = ? 
WHERE "slugs"."name" = 'the-slug'  [[nil, "Post"]]
``` 

It turns out to fix [issue_15821](https://github.com/rails/rails/issues/15821) in a general way is to prepend `relation.arel.bind_values` to `binds` from `DatabaseStatements#binds_from_relation`. 

```ruby 
# /active_record/relation/finder_methods.rb
def exists?(conditions = :none)
  connection.select_value(relation, "#{name} Exists", relation.bind_values) ? true : false
end

# /active_record/connection_adapters/abstract/database_statements.rb
def select_value(arel, name = nil, binds = [])
  # select_one method then call binds_from_relation
  if result = select_one(arel, name, binds)
    result.values.first
  end
end  

def binds_from_relation(relation, binds)
  if relation.is_a?(Relation) && binds.blank?
    # should prepend relation.arel.bind_values to binds
    relation, binds = relation.arel, relation.bind_values
  end
  [relation, binds]
end
```

I try to solve this few days ago, the first commit in [pull request 15861](https://github.com/rails/rails/pull/15861) only fix `FinderMethods#exists?`. Eventually I think put the modification on `DatabaseStatements#binds_from_relation` will better. This help lower level methods  `#select_values` and `#select_all` to transform the `ActiveRecord::Relation` to `Arel` with correct binds. 


Do you know where we have different sets of values on the `relation.arel.bind_values` and `relation.bind_values`? I guess this is a problem of who is building the relation that should prepend the arel bind values when it is built.