fixes missing values in fred() being string

[dwinrow]

Apologies, the previous pull #82 ended up reverting #81 when merged but shouldn't have.

Fixes #79

[codecov]

Codecov Report

Merging #84 (b8bf7eb) into master (cbeb769) will not change coverage.
The diff coverage is 100.00%.

❗ Current head b8bf7eb differs from pull request most recent head 1864f42. Consider uploading reports for the commit 1864f42 to get more accurate results

@@           Coverage Diff           @@
##           master      #84   +/-   ##
=======================================
  Coverage   93.10%   93.10%           
=======================================
  Files           4        4           
  Lines          58       58           
=======================================
  Hits           54       54           
  Misses          4        4           

Impacted Files	Coverage Δ
src/downloads.jl	92.30% <100.00%> (ø)

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