Forces utf8 when no encoding in header for DOI request.

The default behavior from the requests library is to use the guess from
chardet which is not always reliable while doi.org seems to always
return utf8 encoded data. It's unlikely that this changes without also
updating the header.

pubs/apis.py

@@ -3,15 +3,19 @@
 import requests
 from bs4 import BeautifulSoup
 
+
 def doi2bibtex(doi):
     """Return a bibtex string of metadata from a DOI"""
 
     url = 'http://dx.doi.org/{}'.format(doi)
     headers = {'accept': 'application/x-bibtex'}
     r = requests.get(url, headers=headers)
+    if r.encoding is None:
+        r.encoding = 'utf8'  # Do not rely on guessing from request
 
     return r.text
 
+
 def isbn2bibtex(isbn):
     """Return a bibtex string of metadata from a DOI"""