Add specs to test Array#select form of ScopeProxy#select

These tests should fail. The fix is in the next commit.

This should raise:

    ArgumentError: Call this with at least one field

This is because the block is not being passed through.
Fix to allow Array#select form on RelationProxy#select

When `select` is called on an `Octopus::RelationProxy` it is supposed to
delegate to `ActiveRecord::QueryMethods#select`. The ActiveRecord version
takes an optional block, which makes the method behave like
`Array#select`

The block is now passed through and passes the test
Merge pull request #262 from emorikawa/fix-select-block

FIX: Let `Octopus::RelationProxy#select` takes a block so it can be used just like `Array#select`.