oai-pmh-list: fix resumption token handling without attributes

[andrenarchy]

Fixes #7.

[codecov-io]

Codecov Report

Merging #17 into master will increase coverage by 11.39%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #17       +/-   ##
==========================================
+ Coverage    77.1%   88.5%   +11.39%     
==========================================
  Files           5       5               
  Lines          83      87        +4     
  Branches       18      21        +3     
==========================================
+ Hits           64      77       +13     
+ Misses         19      10        -9

Impacted Files	Coverage Δ
src/oai-pmh-list.js	96% <83.33%> (+43.61%)	⬆️

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