Properly extract uploadid from request

Fixes

```
DEBUG [2014-07-10 15:47:21,118] New I/O worker #96 - io.pithos.operations - handling operation:  :delete-object-uploadid
ERROR [2014-07-10 15:47:21,120] New I/O worker #96 - io.pithos.operations - caught exception during operation
java.lang.NullPointerException
        at java.util.UUID.fromString(UUID.java:192)
        at io.pithos.util$parse_uuid.invoke(util.clj:70)
        at io.pithos.operations$abort_upload.invoke(operations.clj:336)
```