fix issue when site returns single result

[VirtueMe]

Fixes paperhive/oai-pmh#28

[VirtueMe]

Strange, det reported line is fixed, but travis still reports error.

Edit: Even stranger, I thought I had fixed it, but the file wasn't changed after all.

[codecov-io]

Codecov Report

Merging #30 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #30   +/-   ##
======================================
  Coverage    88.5%   88.5%           
======================================
  Files           5       5           
  Lines          87      87           
  Branches       21      21           
======================================
  Hits           77      77           
  Misses         10      10

Impacted Files	Coverage Δ
src/oai-pmh-list.js	96% <100%> (ø)	⬆️

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