support data_key

[DeoLeung]

we have some schema using unqualified names like 2019, 1abc, we use data_key to support it

class A(Schema):
  a = fields.String(data_key='2019')

we would like it to be dumped as

{'2019': {'title': '2019', 'type': 'string'}

it's now dumped as

{'a': {'title': 'a', 'type': 'string'}

[fuhrysteve]

Looks like there's been some progress here: #100 but the tests aren't currently passing on that branch

[ZoranPavlovic]

I can pick this up as I'm in need of it. @fuhrysteve - want me to take the other Lundalogik branch to preserve his commits, or start a fresh one with their changes included?

[martijnthe]

Fixed by #139 ?

[rw-access]

I still see this as of marshmallow-jsonschema==0.11.1