Make Enums Nullable By Default

[sigmavirus24]

If you have a definition similar to this:

grade: &grade
  description: Student's final grade
  type:
    - "null"
    - type: string
      enum: [A, B, C, D, F]

Then it is not actually nullable even though you specify "null" as a type.

[sigmavirus24]

Also, it should be noted that the following does not work either:

grade: &grade
   description: Student's final grade
   type: string
   enum: [A, B, C, D, F]
   nullable: true

[myronmarston]

I have a fix in #38. I'll try to update Vanguard to this version later today. Thanks for reporting this!