Fix problem with thread safety in PLSQL::OCIConnection::Cursor

[flash-gordon]

Hi,

I found problem with closing cursors in multithreaded AR MRI environment. It's quite trivial, see changes.

[kubo]

This pull request causes memory leaks. Once @@open_cursors[thread_id] is set, [] is never freed.

flash-gordon@d363756#diff-6cee84182ea605978815af3738661976R72 should be changed as follows:

  def self.open_cursors
    Thread.current[:plsql_oci_cursor_stack] ||= []
  end

[javornikolov]

@kubo, @flash-gordon - are you happy with PR #80 as final solution to the described problem? (I just applied the suggestion for using Thread.current)

[flash-gordon]

@javornikolov thanks, I forget about the PR. Seems to be OK