Chaining scopes can trigger excessive db queries

I was unable to find a reason for the current_scoped_methods_when_defined test in Scope#method_missing. All tests pass without it.

I am attaching an updated patch with a better unit test and that removes the current_scoped_methods_when_defined test.

Thanks for the patch. Went from this

>> Cow.after('a').after('b').after('c').after('d').all
  Cow Load (0.5ms)  SELECT "kine".* FROM "kine" WHERE (name > 'a') AND (name > 'b')
  Cow Load (0.2ms)  SELECT "kine".* FROM "kine" WHERE (name > 'a')
  Cow Load (0.2ms)  SELECT "kine".* FROM "kine" WHERE (name > 'a') AND (name > 'b') AND (name > 'c')
  Cow Load (0.2ms)  SELECT "kine".* FROM "kine" WHERE (name > 'a') AND (name > 'b') AND (name > 'c') AND (name > 'd')
=> [#<Cow id: 1, name: "sam", created_at: "2010-02-25 05:04:04", updated_at: "2010-02-25 05:04:04">]

to this! Happy cows.

>> Cow.after('a').after('b').after('c').after('d').all
  Cow Load (0.3ms)  SELECT "kine".* FROM "kine" WHERE (name > 'a') AND (name > 'b') AND (name > 'c') AND (name > 'd')
=> [#<Cow id: 1, name: "sam", created_at: "2010-02-25 05:04:04", updated_at: "2010-02-25 05:04:04">]