fix: http server codegen

[abocim]

This PR fixes a bug we have hit within http server code generator.

Sibling fields in result types were sharing the same AttributeExpr pointer during projection, causing metadata (descriptions and JSON tags) to leak between them. Added field name to the seen key in projectRecursive to be ensured each sibling gets its own isolated entry.

Goa design code:

...
	var UserType = Type("UserType", func() {
		Attribute("u", Int)
	})

	var RT = ResultType("ResultTypeSibling", func() {
		Attribute("a", UserType, "Attribute A", func() {
			Meta("struct:tag:json", "a")
		})
		Attribute("b", UserType, "Attribute B", func() {
			Meta("struct:tag:json", "b")
		})
	})

	Service("ServiceResultUserTypeSibling", func() {
		Method("MethodResultUserTypeSibling", func() {
			Result(RT)
			HTTP(func() {
				GET("/")
			})
		})
	})
...

Incorrectly generated code with leaked descriptions (comments) and JSON tags (gen/http/v2/server/types.go):

...
// MethodResultUserTypeSiblingResponseBody is the type of the
// "ServiceResultUserTypeSibling" service "MethodResultUserTypeSibling"
// endpoint HTTP response body.
type MethodResultUserTypeSiblingResponseBody struct {
	// Attribute A
	A *UserTypeResponseBody `json:"a"`
	// Attribute A
	B *UserTypeResponseBody `json:"a"`
}

// UserTypeResponseBody is used to define fields on response body types.
type UserTypeResponseBody struct {
	U *int `form:"u,omitempty" json:"u,omitempty" xml:"u,omitempty"`
}
...