The JWTAuth security scheme should get the token from Cookie as well

Currently the JWTAuth security scheme assumes the token from the HTTP header only

With the latest release of adding cookie in the design, the The JWTAuth security scheme should be able to read the token from cookie also

For example

```bash
var _ = Service("rating", func() {
Description("The rating service exposes endpoints to read and write user's rating for resources")

Error("not-found", ErrorResult, "Resource Not Found Error")
Error("internal-error", ErrorResult, "Internal server error")
Error("invalid-token", ErrorResult, "Invalid User token")
Error("invalid-scopes", ErrorResult, "Invalid User scope")

Method("Get", func() {
Description("Find user's rating for a resource")
Security(types.JWTAuth, func() {
Scope("rating:read")
})
Payload(func() {
Attribute("id", UInt, "ID of a resource")
Token("token", String, "JWT")
Required("id", "token")
})
Result(func() {
Attribute("rating", Int, "User rating for resource", func() {
Example("rating", 4)
})
Required("rating")
})

HTTP(func() {
GET("/resource/{id}/rating")
Cookie("token:Authorization")

Response("not-found", StatusNotFound)
Response("internal-error", StatusInternalServerError)
Response("invalid-token", StatusUnauthorized)
Response("invalid-scopes", StatusForbidden)
})
})
})
```

On running the `goa-gen` command it gets executed correctly
but on running the api it gives the following error

```
# github.com/tektoncd/hub/api/gen/http/rating/server
gen/http/rating/server/encode_decode.go:41:4: token redeclared in this block
previous declaration at gen/http/rating/server/encode_decode.go:40:4
gen/http/rating/server/types.go:295:43: duplicate argument token
```