Prevent http.client.InvalidURL exception on set_article_url()

[gbnegrini]

Hi @mfcovington!
I came across an uncaught "http.client.InvalidURL: URL can't contain control characters" exception while running pubmed-lookup on a project.
The issue was caused by a weird record['DOI'] string containing a whitespace (PMID: 31207675). The error can be reproduced running the code below:

from pubmed_lookup import PubMedLookup, Publication
email = ''
pmid = '31207675'
lookup = PubMedLookup(pmid, email)
publication = Publication(lookup, resolve_doi=True)

I've added some split() and join() to set_article_url() method to remove whitespace characters from URLs. I don't think this is such a common issue, but I hope this change can help.