Handle anyOf, allOf, oneOf and not schema keywords

[fenollp]

Fixes 80% of #23

BTW testing needs a great overall. I'll do it using https://github.com/alfert/propcheck when I find the time.
Also, validation is far from correct in many places (this is hard to get right!). See for example the few clauses I had to patch even though they are not related to this PR.

Thanks a lot for creating this package however! :)

[fenollp]

Note WRT enums validation: cast does not "atomify" keys in maps of enum payloads. In other words, the following code does not crash:

schema = %Schema{enum: [%{id: 42}]}
spec = ...
# string keys
{:ok, data} = OpenApiSpex.cast(spec, schema, %{"id" => 42})
{:error, _} = OpenApiSpex.validate(spec, schema, data)
# atom keys
{:ok, data} = OpenApiSpex.cast(spec, schema, %{id: 42})
:ok = OpenApiSpex.validate(spec, schema, data)

I think it would be very dangerous (OOM) to atomify enum keys anyway!
My current workaround is to specify enum schemas with string keys instead, which can be easily missed by anyone.

So how about throwing an error when creating a schema that has atom keys in enum: ...?

[mbuhot]

Thanks for this! Looking pretty good. I'll take a closer read through the code this week.

[mbuhot]

Can you clarify this comment?

From https://swagger.io/docs/specification/data-models/data-types/:

Null
OpenAPI 3.0 does not have an explicit null type as in JSON Schema, but you can use nullable: true to specify that the value may be null.
The example above may be mapped to the nullable types int? in C# and java.lang.Integer in Java.

My interpretation is that the value must match the schema or be null, not anything.

[fenollp]

Yes this is also my interpretation.
However the schema in {"nullable": true} is {} which is the schema that matches anything. Do you want me to remove that comment maybe?

I'd be much happier if we'd use ex_json_schema instead of rolling out our own schema handling...

[mbuhot]

Ah I get it now after reading https://swagger.io/docs/specification/data-models/data-types/#any. Any type is allowed when no type is specified, and nullable: true additionally allows null.

[mbuhot]

I've manually incorporated most of the changes from this PR, and captured the Enum issue in #60.