Use default_public as default api key

[Jesus89]

set_default_context(
    base_url='https://cartovl.carto.com/',
    api_key='default_public'
)

c = Context(
    base_url='https://cartovl.carto.com/',
    api_key='default_public'
)

Should be equivalent to:

set_default_context('https://cartovl.carto.com/')

c = Context('https://cartovl.carto.com/')

What do you think about this proposal @andy-esch

[andy-esch]

Love it! :)