Description of problem:
During importing manifests, the time validation should be done against last imported manifest's creating time, but not the last importing time.

Version-Release number of selected component (if applicable):
katello-headpin-all-0.1.135-2.el6.noarch
katello-cli-headpin-0.1.15-2.el6.noarch
thumbslug-0.0.21-1.el6.noarch
candlepin-0.5.18-1.el6.noarch

How reproducible:
always

Steps to Reproduce:
1.Prepare three manifests which were exported from the same distributor in order :1) manifest1  2) manifest2  3) manifest3
2.Import manifest1 to SAM.
    # headpin -u admin -p admin provider import_manifest --org=org1 --name="Red Hat" --file=manifest1.zip
Manifest imported
3.Import manifest2 to SAM.
    # headpin -u admin -p admin provider import_manifest --org=org1 --name="Red Hat" --file=manifest2.zip
Manifest imported
4.Import manifest3 to SAM.
    # headpin -u admin -p admin provider import_manifest --org=org1 --name="Red Hat" --file=manifest3.zip
  
Actual results:
Importing manifest, please wait... Manifest import for provider [ Red Hat ] failed, Candlepin::Owner: 409 Conflict {"displayMessage":"Import is older than existing data"} (POST /candlepin/owners/org_test/imports)

Expected results:
As manifest3 is newer than existing one(manifest2), it should be imported successfully.

Additional info:
1.metadata of these manifests:
    1)manifest1 {"version":"0.0.0","created":"2011-12-13T13:39:18.111+0000"} 
    2)manifest2 {"version":"0.5.5-1","created":"2012-01-09T02:45:23.545+0000"}
    3)manifest3 {"version":"0.5.5-1","created":"2012-01-12T02:14:21.037+0000"}
2.related candlepin code
    candlepin/sync/Importer.java
public void validateMetadata(String type, Owner owner, File meta, boolean force)
        throws IOException, ImporterException {
        ......

        if (lastrun == null) {
            // this is our first import, let's create a new entry
            lastrun = new ExporterMetadata(type, m.getCreated(), owner);
            lastrun = expMetaCurator.create(lastrun);
        }
        else {
            if (!force && lastrun.getExported().compareTo(m.getCreated()) >= 0){
                throw new ConflictException(i18n.tr("Import is older than existing data"));
            }
            else {
                lastrun.setExported(new Date());
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^shoud be set to creating time of the imported manifest
                expMetaCurator.merge(lastrun);
            }
        }
    }