Enable using Arel nodes in Squeel with "|" operator.

Fixes #314

Examples:

    User.where {
      (id.in([1,2,3]) | User.arel_table[:id].eq(1)) &
      ((id == 1) | User.arel_table[:id].not_eq(nil)) }
Add a spec to prove the problem of #302 had gone.
Merge pull request #319 from bigxiang/fix-314

Enable using Arel nodes in Squeel with "|" operator.
Add a spec to prove the problem of #302 had gone.