Fix array of additional flags

[CAPSLOCK2000]

Do not ignore the 'additional_flags' option of docker_networks when it is specified as an array instead of a simple string.
Without this fix 'additional_flags' is silently ignored when it is an array.

[codecov-io]

Codecov Report

Merging #671 into main will increase coverage by 0.40%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #671      +/-   ##
==========================================
+ Coverage   22.19%   22.60%   +0.40%     
==========================================
  Files          20       20              
  Lines         757      761       +4     
==========================================
+ Hits          168      172       +4     
  Misses        589      589              

Impacted Files	Coverage Δ
lib/puppet/provider/docker_network/ruby.rb	53.84% <66.66%> (+3.84%)	⬆️

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