[BUGFIX] Merge request arguments instead of overwrite (Revision 1)

A sub-request losing its arguments if a route is defined f.e. like this:
-
  name: 'Basket Steps'
  uriPattern: '{node}/{--pipeu_factura_nodetypes-basket.@action}.{@format}'
  defaults:
    '@package': 'TYPO3.Neos'
    '@controller': 'Frontend\Node'
    '@format': 'html'
    '@action': 'show'
    '--pipeu_factura_nodetypes-basket':
      '@package': 'PIPEU.Factura.NodeTypes'
      '@controller': 'Basket'
  routeParts:
    node:
      handler: TYPO3\Neos\Routing\FrontendNodeRoutePartHandler
  appendExceedingArguments: true

In this case '--pipeu_factura_nodetypes-basket' will override
any pluginArgument because of a missing check for existing
index in pluginArguments. Its getting replaced instead of merged.

This wrong behavior is fixed with this patch by merging instead of
overwriting.

Same for internal arguments.

Was fixed in master by the related feature.

Related: I1d192231810757f38aecfc3f7dc520b118ee3feb
Change-Id: Ib067912327cb07cc6fff91d1fe5b3e5c6cd9ccdb
Resolves: #59404
Releases: master, 2.2, 2.1
Revision-Id: 7e643f7b00ed624b8385f5f02f2862eaba43d40c