`_` considered as invalid char in package names

according to [this](https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html) `_` is a valid char in package names but adding following line in `io.vertx.test.codegen.CaseTest#testParseQualifiedCase`:

```java
parseQualifiedCase("foo.bar.juu_oo", "foo", "bar", "juu_oo");
```

will result in a failed test result:

```
-------------------------------------------------------------------------------
Test set: io.vertx.test.codegen.CaseTest
-------------------------------------------------------------------------------
Tests run: 10, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.001 sec <<< FAILURE! - in io.vertx.test.codegen.CaseTest
testParseQualifiedCase(io.vertx.test.codegen.CaseTest) Time elapsed: 0.001 sec <<< ERROR!
java.lang.IllegalArgumentException: Invalid qualified case:foo.bar.juu_oo
at io.vertx.codegen.QualifiedCase.parse(QualifiedCase.java:35)
at io.vertx.test.codegen.CaseTest.parseCase(CaseTest.java:178)
at io.vertx.test.codegen.CaseTest.parseQualifiedCase(CaseTest.java:166)
at io.vertx.test.codegen.CaseTest.testParseQualifiedCase(CaseTest.java:65)
```