Support harvest dataset source identifiers that ends with a '/'

[rhabbachi]

Harvest sources with datasets identifier fields that ends with "/" (for example http://dx.doi.org/10.7927/H4PZ56R2/) will result in an empty generated identifier during the Harvest.

This PR tweaks the dkan_harvest_datajson_prepare_item_id function to account for this case and return the last non empty segment of the identifier.

How to reproduce

1. Harvest a source that have a dataset with an identifier that is a URL that ends with a "/" (in the following example the identifier is "http://demo.getdkan.com/90a2b708-7fea-4b92-8aee-43c4cfdd5f48/").

        {
            "@type": "dcat:Dataset",
            "accessLevel": "public",
            "contactPoint": {
                "fn": "admin",
                "hasEmail": "mailto:admin@example.com"
            },
            "description": "<p>Polling places in the state of Wisconsin</p>",
            "distribution": [
                {
                    "@type": "dcat:Distribution",
                    "description": "No description provided",
                    "downloadURL": "https://s3.amazonaws.com/dkan-default-content-files/phpunit/Polling_Places_Madison_0.csv",
                    "format": "csv",
                    "mediaType": "text/csv"
                }
            ],
            "identifier": "http://demo.getdkan.com/90a2b708-7fea-4b92-8aee-43c4cfdd5f48/",
            "keyword": [
                "election"
            ],
            "language": [
                "en"
            ],
            "license": "http://opendefinition.org/licenses/cc-by/",
            "modified": "2015-07-11",
            "publisher": {
                "@type": "org:Organization",
                "name": "Geospatial Data Explorer Examples"
            },
            "title": "Wisconsin Polling Places TEST"
        },

Expected Behavior:
Harvest works.

Current Behavior:
Harvest fail and identifier recorded is empty.

QA Steps

• Following the How to reproduce section steps should result in the source harvested with no errors.

Merge process

• N/A.

Reminders

• There is test for the issue.
• Coding standards checked.
• Review docs.getdkan.com (or in /docs) to see if it still covers the scope of the PR and update if needed.