fix URLs for GB public bodies (issue #97)

[augusto-herrmann]

It now passes Goodtables validation.

$ goodtables --schema public-body-schema.json data/gb.csv
DATASET
=======
{'error-count': 0,
 'preset': 'nested',
 'table-count': 1,
 'time': 1.724,
 'valid': True}

TABLE [1]
=========
{'encoding': 'cp1252',
 'error-count': 0,
 'format': 'csv',
 'headers': ['id',
             'name',
             'abbreviation',
             'other_names',
             'description',
             'classification',
             'parent_id',
             'founding_date',
             'dissolution_date',
             'image',
             'url',
             'jurisdiction_code',
             'email',
             'address',
             'contact',
             'tags',
             'source_url'],
 'row-count': 5363,
 'schema': 'table-schema',
 'scheme': 'file',
 'source': 'data/gb.csv',
 'time': 1.721,
 'valid': True}

This should solve #97.

[rufuspollock]

@augusto-herrmann please feel free to self-merge your PRs - i don't want you to get blocked and you are doing awesome 🍕 🏆 work here :smile:

[augusto-herrmann]

Thanks, @rufuspollock! :smile: :sparkles: It is usually considered good practice to have someone else review the PRs, just in case I do not spot some error. But I realize people are not always available to review. So I think I'll continue doing the PRs, and if after a few days someone doesn't review, I'll go ahead and self-merge.