scep: minor cleanup (smallstep#867)

* api, scep: removed scep.Error

* scep/api: replaced nextHTTP with http.HandlerFunc

* scep/api: renamed writeSCEPResponse to writeResponse

* scep/api: renamed decodeSCEPRequest to decodeRequest

* scep/api: renamed writeError to fail

* scep/api: replaced pkg/errors with errors

* scep/api: formatted imports

* scep/api: do not export SCEPRequest & SCEPResponse

* scep/api: do not export Handler

* api: flush errors better

Author: azazeal

Diff for: api/errors.go

@@ -13,7 +13,6 @@ import (
 	"github.com/smallstep/certificates/authority/admin"
 	"github.com/smallstep/certificates/errs"
 	"github.com/smallstep/certificates/logging"
-	"github.com/smallstep/certificates/scep"
 )
 
 // WriteError writes to w a JSON representation of the given error.
@@ -25,22 +24,9 @@ func WriteError(w http.ResponseWriter, err error) {
 	case *admin.Error:
 		admin.WriteError(w, k)
 		return
-	case *scep.Error:
-		w.Header().Set("Content-Type", "text/plain")
-	default:
-		w.Header().Set("Content-Type", "application/json")
 	}
 
 	cause := errors.Cause(err)
-	if sc, ok := err.(errs.StatusCoder); ok {
-		w.WriteHeader(sc.StatusCode())
-	} else {
-		if sc, ok := cause.(errs.StatusCoder); ok {
-			w.WriteHeader(sc.StatusCode())
-		} else {
-			w.WriteHeader(http.StatusInternalServerError)
-		}
-	}
 
 	// Write errors in the response writer
 	if rl, ok := w.(logging.ResponseLogger); ok {
@@ -60,6 +46,16 @@ func WriteError(w http.ResponseWriter, err error) {
 		}
 	}
 
+	code := http.StatusInternalServerError
+	if sc, ok := err.(errs.StatusCoder); ok {
+		code = sc.StatusCode()
+	} else if sc, ok := cause.(errs.StatusCoder); ok {
+		code = sc.StatusCode()
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(code)
+
 	if err := json.NewEncoder(w).Encode(err); err != nil {
 		log.Error(w, err)
 	}