[HOTFIX] Collection.scroll: the scroll parameter is now optional

[scottinet]

Description

Following kuzzleio/kuzzle#709, the scroll option is now optional (!)
Also fix the way the query is built to make it consistent with the way kuzzle.query is meant to be used.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3c3150f). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #192   +/-   ##
=========================================
  Coverage          ?   99.45%           
=========================================
  Files             ?       16           
  Lines             ?     1663           
  Branches          ?      440           
=========================================
  Hits              ?     1654           
  Misses            ?        9           
  Partials          ?        0

Impacted Files	Coverage Δ
src/Collection.js	97.33% <100%> (ø)

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