Rely on rpcserver.DefaultConfig() to inherit default config values

[alessio]

This fixes a small regression inadvertently introduced
in 99df748 that caused the REST server to return
"http: request body too large" on POST requests handling.

closes: #4845

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• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry: clog add [section] [-t <tag>] [-m <msg>]
• Re-reviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[alexanderbez]

ACK

[codecov]

Codecov Report

Merging #4848 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4848   +/-   ##
=======================================
  Coverage   54.11%   54.11%           
=======================================
  Files         269      269           
  Lines       17216    17216           
=======================================
  Hits         9317     9317           
  Misses       7209     7209           
  Partials      690      690