doc: fix added: info for some methods

`outgoingMessage.getHeader()`, `outgoingMessage.getHeaderNames()`, and
`outgoingMessage.getHeaders()` were added to Node.js v7.7.0 via
3e8d43d.

PR-URL: nodejs#42661
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Qingyu Deng <i@ayase-lab.com>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>

doc/api/http.md

@@ -2579,7 +2579,7 @@ exist in message, it will be `undefined`.
 ### `outgoingMessage.getHeaderNames()`
 
 <!-- YAML
-added: v8.0.0
+added: v7.7.0
 -->
 
 * Returns {string\[]}
@@ -2590,7 +2590,7 @@ names are lowercase.
 ### `outgoingMessage.getHeaders()`
 
 <!-- YAML
-added:  v8.0.0
+added: v7.7.0
 -->
 
 * Returns: {Object}
@@ -2617,7 +2617,7 @@ const headers = outgoingMessage.getHeaders();
 ### `outgoingMessage.hasHeader(name)`
 
 <!-- YAML
-added:  v8.0.0
+added: v7.7.0
 -->
 
 * `name` {string}