Don't fail if writing IO doesn't support seek. Fixes #544

[quinnj]

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a5a29a0). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #545   +/-   ##
=========================================
  Coverage          ?   83.26%           
=========================================
  Files             ?        7           
  Lines             ?     1434           
  Branches          ?        0           
=========================================
  Hits              ?     1194           
  Misses            ?      240           
  Partials          ?        0

Impacted Files	Coverage Δ
src/write.jl	87.5% <100%> (ø)

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