Fix unscope when an eq node which has no arel attribute

[kamipo]

Current code expect an eq node has one arel attribute at least, but an
eq node may have no arel attribute (e.g. Arel.sql("...").eq(...)).

In that case unscope will raise NoMethodError:

% bundle exec ruby -w -Itest test/cases/relations_test.rb -n test_unscope_with_arel_sql
Using sqlite3
Run options: -n test_unscope_with_arel_sql --seed 4477

# Running:

E

Error:
RelationTest#test_unscope_with_arel_sql:
NoMethodError: undefined method `name' for #<Arel::Nodes::Quoted:0x00007f9938e55960>
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/where_clause.rb:157:in `block (2 levels) in except_predicates'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/arel.rb:57:in `fetch_attribute'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/where_clause.rb:157:in `block in except_predicates'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/where_clause.rb:156:in `reject'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/where_clause.rb:156:in `except_predicates'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/where_clause.rb:31:in `except'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/query_methods.rb:487:in `block (2 levels) in unscope!'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/query_methods.rb:481:in `each'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/query_methods.rb:481:in `block in unscope!'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/query_methods.rb:471:in `each'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/query_methods.rb:471:in `unscope!'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/relation/query_methods.rb:464:in `unscope'
    test/cases/relations_test.rb:2062:in `test_unscope_with_arel_sql'

We should check for both value.left and value.right those are arel
attribute or not.