use oneunit, not one, to preserve type in single_seed

[stevengj]

Fixes #651

This change is also required by #664, so if that PR is accepted then it supersedes this PR.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -3.07% :warning:

Comparison is base (d300209) 89.65% compared to head (1db2302) 86.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #665      +/-   ##
==========================================
- Coverage   89.65%   86.59%   -3.07%     
==========================================
  Files          11       10       -1     
  Lines         967      895      -72     
==========================================
- Hits          867      775      -92     
- Misses        100      120      +20     

Files Changed	Coverage Δ
src/partials.jl	84.07% <100.00%> (-0.14%)	:arrow_down:

... and 9 files with indirect coverage changes

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