Introduce limits middleware

1. Replace original `maxrequestbody` directive.
2. Add request header limit.

fix issue #1587

Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/limits/handler_test.go

@@ -0,0 +1,35 @@
+package limits
+
+import (
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/mholt/caddy/caddyhttp/httpserver"
+)
+
+func TestBodySizeLimit(t *testing.T) {
+	var (
+		gotContent    []byte
+		gotError      error
+		expectContent = "hello"
+	)
+	l := Limit{
+		Next: httpserver.HandlerFunc(func(w http.ResponseWriter, r *http.Request) (int, error) {
+			gotContent, gotError = ioutil.ReadAll(r.Body)
+			return 0, nil
+		}),
+		BodyLimits: []httpserver.PathLimit{{Path: "/", Limit: int64(len(expectContent))}},
+	}
+
+	r := httptest.NewRequest("GET", "/", strings.NewReader(expectContent+expectContent))
+	l.ServeHTTP(httptest.NewRecorder(), r)
+	if got := string(gotContent); got != expectContent {
+		t.Errorf("expected content[%s], got[%s]", expectContent, got)
+	}
+	if gotError != httpserver.MaxBytesExceededErr {
+		t.Errorf("expect error %v, got %v", httpserver.MaxBytesExceededErr, gotError)
+	}
+}