Avoid replacing DOIs with shorter ones

[lfoppiano]

When we search for a DOI in the page, the regex may truncate DOIs that are split by a breakline, so this PR proposes a simple fix that is to substitute the DOI only when the one found in the page is larger than the one extracted by the header parser

[coveralls]

coverage: 40.786% (-0.001%) from 40.787%
when pulling 7f1d15d on bugfix/fix-doi-search
into 694f0ed on master.

[coveralls]

coverage: 40.396% (+0.002%) from 40.394%
when pulling 7f5b55e on bugfix/fix-doi-search
into 01fe109 on master.