Revert "http: headers(Distinct), trailers(Distinct) setters to be no-op"

This reverts commit 4d723c7.

I'm not sure if we should re-apply this as a semver-major change or if
we should accept it as valid and add tests/documentation, but either
way, we have to revert it at least temporarily.

Closes: nodejs#45510

lib/_http_incoming.js

@@ -122,7 +122,9 @@ ObjectDefineProperty(IncomingMessage.prototype, 'headers', {
     }
     return this[kHeaders];
   },
-  set: function(val) {}
+  set: function(val) {
+    this[kHeaders] = val;
+  }
 });
 
 ObjectDefineProperty(IncomingMessage.prototype, 'headersDistinct', {
@@ -140,7 +142,9 @@ ObjectDefineProperty(IncomingMessage.prototype, 'headersDistinct', {
     }
     return this[kHeadersDistinct];
   },
-  set: function(val) {}
+  set: function(val) {
+    this[kHeadersDistinct] = val;
+  }
 });
 
 ObjectDefineProperty(IncomingMessage.prototype, 'trailers', {
@@ -158,7 +162,9 @@ ObjectDefineProperty(IncomingMessage.prototype, 'trailers', {
     }
     return this[kTrailers];
   },
-  set: function(val) {}
+  set: function(val) {
+    this[kTrailers] = val;
+  }
 });
 
 ObjectDefineProperty(IncomingMessage.prototype, 'trailersDistinct', {
@@ -176,7 +182,9 @@ ObjectDefineProperty(IncomingMessage.prototype, 'trailersDistinct', {
     }
     return this[kTrailersDistinct];
   },
-  set: function(val) {}
+  set: function(val) {
+    this[kTrailersDistinct] = val;
+  }
 });
 
 IncomingMessage.prototype.setTimeout = function setTimeout(msecs, callback) {

test/parallel/test-set-incoming-message-header.js

@@ -4,23 +4,24 @@ require('../common');
 const { IncomingMessage } = require('http');
 const assert = require('assert');
 
-// Headers setter should be a No-Op
+// Headers setter function set a header correctly
 {
   const im = new IncomingMessage();
   im.headers = { key: 'value' };
-  assert.deepStrictEqual(im.headers, {});
+  assert.deepStrictEqual(im.headers, { key: 'value' });
 }
 
-// Trailers setter should be a No-Op
+// Trailers setter function set a header correctly
 {
   const im = new IncomingMessage();
   im.trailers = { key: 'value' };
-  assert.deepStrictEqual(im.trailers, {});
+  assert.deepStrictEqual(im.trailers, { key: 'value' });
 }
 
 // _addHeaderLines function set a header correctly
 {
   const im = new IncomingMessage();
+  im.headers = { key1: 'value1' };
   im._addHeaderLines(['key2', 'value2'], 2);
-  assert.deepStrictEqual(im.headers, { key2: 'value2' });
+  assert.deepStrictEqual(im.headers, { key1: 'value1', key2: 'value2' });
 }