Make the `Target` optional in `Challenge` object

This is a non-standard property in the ACME challenge response, so
we shouldn't return it if it's not set. Also made it an optional
field in the DB.

acme/challenge.go

@@ -82,7 +82,7 @@ type Challenge struct {
 	Token           string        `json:"token"`
 	ValidatedAt     string        `json:"validated,omitempty"`
 	URL             string        `json:"url"`
-	Target          string        `json:"target"`
+	Target          string        `json:"target,omitempty"`
 	Error           *Error        `json:"error,omitempty"`
 }
 

acme/db/nosql/challenge.go

@@ -19,7 +19,7 @@ type dbChallenge struct {
 	Status      acme.Status        `json:"status"`
 	Token       string             `json:"token"`
 	Value       string             `json:"value"`
-	Target      string             `json:"target"`
+	Target      string             `json:"target,omitempty"`
 	ValidatedAt string             `json:"validatedAt"`
 	CreatedAt   time.Time          `json:"createdAt"`
 	Error       *acme.Error        `json:"error"` // TODO(hs): a bit dangerous; should become db-specific type