issue #15 - fix inner Map classes serialization

[wajda]

Fixes #15

Adds support for more map types, specifically anonymous ones that are instantiated when calling methods like:

• Map.withDefault() or Map.withDefaultValue()
• Map.filterKeys()
• Map.mapValues()

The fix was tested with Scala 2.11 and 2.12

[wajda]

The build fails with Scala 2.13 - it requires a separate treatment as 2.13 has braking changes. The current project structure doesn't accommodate Scala cross-compilation. How should I go about it? Do I need to create a separate PR to every release/velocypack-module-scala_2.XY branch, plus one PR to the master for Scala 2.13?

[rashtao]

With the current project structure every Scala version has a different branch:

branch	scala version
master	2.13
release/velocypack-module-scala_2.12	2.12
release/velocypack-module-scala_2.11	2.11

Due to the nature of this library, we decided to keep different branches for each different Scala version and avoid cross compilation to different versions. This approach requires a bit more work for merging code changes to all branches, but it is safer and easier to maintain with respect to differences in collection API across many Scala versions.

Therefore if there are differences in the code across different Scala versions, you should create a PR for each branch.

[wajda]

Thanks for the info, I'm closing this PR and will create three others respectively.