Improper Mixin warning when using mixins in Attributes without type

[pksunkara]

# GET /
+ Attributes
    + Include A

# Data Structures
## A (object)
+ a: a

Output

Gives warning mixin base type should be the same as parent base type. objects should contain object mixins. arrays should contain array mixins

Expected

Should not give any warning

[pksunkara]

Also related apiaryio/protagonist#133