perform deep merge for Defaults.set()

[jtdevos]

This pull request includes a

• Bug fix
• New feature
• Translation

The following changes were made

• Changed Defaults.set() behavior to perform a "deep" merge, so that it is possible, for example, to change multiple ajax defaults (e.g. "ajax.dataType" and "ajax.delay") without one call overwriting the other.
• Added test to assert new behavior

[jtdevos]

I notice that the build failed - I assume it's because I failed to run grunt prior to committing. Let me know if you'd like me to run grunt and then add the updated dist files to this pull request.

[kevin-brown]

You're missing a comma here, and that is failing the build.

[kevin-brown]

Let me know if you'd like me to run grunt

Running grunt will run the automated tests and confirm that the files can be properly compiled. The CI will automatically do this, so it's not required, but it's helpful to ensure the build doesn't fail when we test it.

and then add the updated dist files to this pull request

This isn't required (and I usually don't encourage it). The CI will automatically compile the files before testing, and including the dist changes usually ends with unnecessary merge conflicts later.

[kevin-brown]

This should really be ajax--data-type, to match the HTML attribute.

[alexweissman]

Merged into develop for 4.0.6 release.