Use json string for collection description

[adelavega]

Merged #632 too soon. requests can pass dictionary, so i'm going to try dumping as string, and reading that in on the server side.

[adelavega]

Great, looks like that works, will merge when tests pass.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@82fcdd2). Click here to learn what that means.
The diff coverage is 33.33%.

@@            Coverage Diff            @@
##             master     #634   +/-   ##
=========================================
  Coverage          ?   82.22%           
=========================================
  Files             ?       57           
  Lines             ?     2183           
  Branches          ?        0           
=========================================
  Hits              ?     1795           
  Misses            ?      388           
  Partials          ?        0

Impacted Files	Coverage Δ
neuroscout/resources/predictor.py	88.46% <33.33%> (ø)

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