Fix bug in tgt caching
Fix bug in tgt caching

This commit fixes a bug in the caching mechanism at
`Cassette::Client#st_with_retry` introduced at 183e1e5a33e7.

As you can see, in our testing suite we have no tests for caching:

```
Pending: (Failures listed here are expected and do not affect your suite's status)

  1) Cassette::Authentication::Cache
     # Not yet implemented
     # ./spec/cassette/authentication/cache_spec.rb:7

  2) Cassette::Client::Cache
     # Not yet implemented
     # ./spec/cassette/client/cache_spec.rb:6
```

That's probably why the bug slipped through our refactoring.

Before this fix:

    irb(main):001:0> Cassette::Client.st_for('hodor')
    fetching from tgt source
    fetching from st source
    => "ST-25146147-Ethe5cFv2vSdZQBI0j3u-moe"
    irb(main):002:0> Cassette::Client.st_for('hodor')
    fetching from tgt source
    fetching from st source
    => "ST-25146147-Ethe5cFv2vSdZQBI0j3u-moe"
    irb(main):003:0> Cassette::Client.st_for('hodor')
    fetching from tgt source
    fetching from st source
    => "ST-25146147-Ethe5cFv2vSdZQBI0j3u-moe"

Which reads: "every time we call #st_for we are fetching a new tgt and
st tickets."

After this fix:

    irb(main):001:0> Cassette::Client.st_for('hodor')
    fetching from tgt source
    fetching from st source
    => "ST-25146147-Ethe5cFv2vSdZQBI0j3u-moe"
    irb(main):002:0> Cassette::Client.st_for('hodor')
    => "ST-25146147-Ethe5cFv2vSdZQBI0j3u-moe"
    irb(main):003:0> Cassette::Client.st_for('hodor')

Which reads: we are not messing everything up with lame refactorings.

locaweb/cassette