The $@ option in bash doesn't make sense to come before `generate`
because the only option we can pass before generate cli usage is `help`.
System properties can be passed via JAVA_OPTS, so there's not really a
need for any intermediaries in the command line construction.
Having $@ at the end of the arguments list allows maintainers and users
inspecting options to quickly pass new options to a script. For example,
```
./bin/aspnetcore-petstore.sh --additional-properties sourceFolder=asdf
```
For command line arguments that may appear more than once in the
arguments list, this change doesn't provide any rules about overwriting
values that may exist (hard-coded) in the script. That is, in the
example above, if aspnetcore-petstore.sh already includes the
sourceFolder set to a different value, the "winning" value is up to the
options parser and openapi-generator-cli implementation.
This change adds Objective-C compatibility for Swift Optional scalars in the swift4 language just like we added in swift3 here:
https://github.com/swagger-api/swagger-codegen/pull/6129
It also adds unit tests for the swift4 language. Currently those unit tests are the same as the swift3 unit tests, but will change soon as we iterate on the swift4 language generator.
