RFR: 2149: Allow multiple query parameters with the same key

[Erik Duveblad]

Hi all.

please review this patch that makes `RestRequest` support multiple query parameters with the same key. This idiom is sometimes used in REST APIs (for example GitLab's REST API). The query parameters should always be kept in program order, since some REST APIs (e.g. GitLab) care about the order of the query parameters.

I also added a new `build` method on `RestRequest` to ease unit testing.

### Testing
- [x] Added a bunch of unit tests

-------------

Commit messages:
 - 2149

Changes: https://git.openjdk.org/skara/pull/1601/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1601&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2149
  Stats: 88 lines in 6 files changed: 76 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/skara/pull/1601.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1601/head:pull/1601

PR: https://git.openjdk.org/skara/pull/1601