* Partially revert "replace deprecated ISO8601Utils with StdDateFormat (#17052)"
This partially reverts commit 76560e34c9aacd9d7593ac45bd204e2edf38abd9, namely anything
related to generators and samples using GSON instead of Jackson.
Changes to Jackson-only generation and generator-online regarding RFC3339DateFormat
are not being reverted.
* Test for default serialization library fallback
* Convert repetitive tests to parameterized test
* Add regression test for #18515
* [FEIGN] Only include <jackson-databind-version> property in pom.xml when required
* [RETROFIT2] Only include jackson-databind in gradle file when actually required
* [FEIGN] Don't include jackson dep's in sbt file when GSON is selected
* [FEIGN] Don't include jackson dep's in gradle file when GSON is selected
* DRY refactor JavaClientCodegen test code, increase readability
- use fluent assertions
- use helper method newTempFolder()
- use Java 9 static factory methods for maps
- don't declare variables that are only used once
- group declarations and usages
- use non-blocking java.nio.file API wherever possible
* Regenerate samples
ed2aad6756