[java][okhttp-gson] support multiple file upload

[lschwahn]

fixes #6134

The method buildRequestBodyMultipart in ApiClient.java now recognizes if an input parameter is an instance of List. So it is possible for clients so send multiple files in one request.

Tested by using the generated code in one of our webservices.

Since the existing Pull-Request 8197 seems to be stuck, we made this one to apply the workaround (fixing the generated code) we are using for more than two years now.

PR checklist

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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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[wing328]

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

[wing328]

@lschwahn thanks for the PR, which has been merged into the master.

[lschwahn]

@wing328 Thanks! I think PR #8197 can be closed, because it fixes the same issue (#6134).