Citation mention linked to bib entry without key

See `paper_id=15213255`:

```
{'section': 'Automating the search',
'text': '(25) The judge announced that the defendant was guilty. Many classes of verbs have already been identified and are incorporated into the system (Nairn et al., 2006) : verbs relating to speech ($
.g., say, report, etc.), implicative verbs such as manage and fail (Karttunen, 2007) , and factive verbs (e.g. agree, realize, consider) (Vendler, 1967; Kiparsky and Kiparsky, 1971) , to name a few. Many
adjectives have also been added to the system, including ones taking to and that complements. 10 As with the complement-taking nouns, a significant part of the effort in incorporating the complement-taki$
g adjectives into the system was identifying which adjectives license complements. The adverbs have not been explored in as much depth.',
'cite_spans': [{'start': 144,
'end': 164,
'text': '(Nairn et al., 2006)',
'ref_id': 'BIBREF16',
'arxiv_id': None,
'paper_id': '525764'},
{'start': 261,
'end': 278,
'text': '(Karttunen, 2007)',
'ref_id': 'BIBREF10',
'arxiv_id': None},
{'start': 347,
'end': 375,
'text': 'Kiparsky and Kiparsky, 1971)',
'ref_id': 'BIBREF11',
'arxiv_id': None}],
```

See `BIBREF11`, yet:

```
citing_paper_dict['pdf_parse']['bib_entries'].keys()
dict_keys(['BIBREF0', 'BIBREF1', 'BIBREF2', 'BIBREF3', 'BIBREF4', 'BIBREF5', 'BIBREF6', 'BIBREF7', 'BIBREF8', 'BIBREF9', 'BIBREF10', 'BIBREF12', 'BIBREF13', 'BIBREF14', 'BIBREF15', 'BIBREF16', 'BIBREF17'])
```

Will need to double-check where it went.