headers: Support default header values in Caddyfile with '?' (#3807)

* implement default values for header directive

closes #3804

* remove `set_default` header op and rely on "require" handler instead

This has the following advantages over the previous attempt:

- It does not introduce a new operation for headers, but rather nicely
  extends over an existing feature in the header handler.
- It removes the need to specify the header as "deferred" because it is
  already implicitely deferred by the use of the require handler. This
  should be less confusing to the user.

* add integration test for header directive in caddyfile

* bubble up errors when parsing caddyfile header directive

* don't export unnecessarily and don't canonicalize headers unnecessarily

* fix response headers not passed in blocks

* caddyfile: fix clash when using default header in block

Each header is now set in a separate handler so that it doesn't clash
with other headers set/added/deleted in the same block.

* caddyhttp: New idle_timeout default of 5m

* reverseproxy: fix random hangs on http/2 requests with server push (#3875)

see golang/go#42534

* Refactor and cleanup with improvements

* More specific link

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>
Co-authored-by: Денис Телюх <telyukh.denis@gmail.com>

caddyconfig/httpcaddyfile/directives.go

@@ -103,20 +103,11 @@ func RegisterHandlerDirective(dir string, setupFunc UnmarshalHandlerFunc) {
 			return nil, h.ArgErr()
 		}
 
-		matcherSet, ok, err := h.MatcherToken()
+		matcherSet, err := h.ExtractMatcherSet()
 		if err != nil {
 			return nil, err
 		}
-		if ok {
-			// strip matcher token; we don't need to
-			// use the return value here because a
-			// new dispenser should have been made
-			// solely for this directive's tokens,
-			// with no other uses of same slice
-			h.Dispenser.Delete()
-		}
 
-		h.Dispenser.Reset() // pretend this lookahead never happened
 		val, err := setupFunc(h)
 		if err != nil {
 			return nil, err
@@ -201,7 +192,12 @@ func (h Helper) ExtractMatcherSet() (caddy.ModuleMap, error) {
 		return nil, err
 	}
 	if hasMatcher {
-		h.Dispenser.Delete() // strip matcher token
+		// strip matcher token; we don't need to
+		// use the return value here because a
+		// new dispenser should have been made
+		// solely for this directive's tokens,
+		// with no other uses of same slice
+		h.Dispenser.Delete()
 	}
 	h.Dispenser.Reset() // pretend this lookahead never happened
 	return matcherSet, nil

caddytest/integration/caddyfile_adapt/header.txt

@@ -0,0 +1,107 @@
+:80 {
+	header Denis "Ritchie"
+	header +Edsger "Dijkstra"
+	header ?John "von Neumann"
+	header -Wolfram
+	header {
+		Grace: "Hopper"  # some users habitually suffix field names with a colon
+		+Ray "Solomonoff"
+		?Tim "Berners-Lee"
+		defer
+	}
+}
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":80"
+					],
+					"routes": [
+						{
+							"handle": [
+								{
+									"handler": "headers",
+									"response": {
+										"set": {
+											"Denis": [
+												"Ritchie"
+											]
+										}
+									}
+								},
+								{
+									"handler": "headers",
+									"response": {
+										"add": {
+											"Edsger": [
+												"Dijkstra"
+											]
+										}
+									}
+								},
+								{
+									"handler": "headers",
+									"response": {
+										"require": {
+											"headers": {
+												"John": null
+											}
+										},
+										"set": {
+											"John": [
+												"von Neumann"
+											]
+										}
+									}
+								},
+								{
+									"handler": "headers",
+									"response": {
+										"deferred": true,
+										"delete": [
+											"Wolfram"
+										]
+									}
+								},
+								{
+									"handler": "headers",
+									"response": {
+										"add": {
+											"Ray": [
+												"Solomonoff"
+											]
+										},
+										"deferred": true,
+										"set": {
+											"Grace": [
+												"Hopper"
+											]
+										}
+									}
+								},
+								{
+									"handler": "headers",
+									"response": {
+										"require": {
+											"headers": {
+												"Tim": null
+											}
+										},
+										"set": {
+											"Tim": [
+												"Berners-Lee"
+											]
+										}
+									}
+								}
+							]
+						}
+					]
+				}
+			}
+		}
+	}
+}