Fix handling of free-form query parameters with Java Resttemplate

[gpspatacean]

Fix for (#8352).
On codegen level, it seems the parsing of the spec file is correct.
The problem seems to be on the template level: the method ApiClient.parameterToMultiValueMap(...), although receives the free form map as parameter, it doesn't have any kind of handling for it.
Input spec file for repro/validation and details on the ticket.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[gpspatacean]

Tech comm:
@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) @nmuesch (2021/01)

And @nbruno for Resttemplate ownership

[gpspatacean]

These changes were merged under #10428 . Closing this one.