fix required struct tag when anonymous nested struct

[RedBoneZhang]

trafficstars

when use anonymous nested structures at @param body，required stuct tag is invalid. struct define like follow:

type StructA struct {
   Name string `json:"name" required:"true"`
}

type StructB struct {
  StructA 
}

error: when use StructB in @param，required struct tag is not generated in swagger.json.

fixed: #779