Change enum to json by a call to name() instead of toString()

[ruget]

Fix proposal for https://github.com/vert-x3/issues/issues/445

*ServiceVertxEBProxy is generated with:

• .valueOf() to marshall from json msg
• .toString() to unmarshall to json msg

If .toString() is overwritten, we are losing the symmetry. So change it by name().

[vietj]

looks straightforward enough to not require a test

[vietj]

this should be back-ported to 3.7

[slinkydeveloper]

@ruget thank you so much for your contribution, I've changed the base to merge to master. I will take care of merging it