doc: fix added: info for some methods

`outgoingMessage.getHeader()`, `outgoingMessage.getHeaderNames()`, and
`outgoingMessage.getHeaders()` were added to Node.js v8.0.0 and
backported to Node.js v7.7.0 via 3e8d43d.

doc/api/http.md

@@ -1659,7 +1659,9 @@ const setCookie = response.getHeader('set-cookie');
 ### `response.getHeaderNames()`
 
 <!-- YAML
-added: v7.7.0
+added:
+  - v8.0.0
+  - v7.7.0
 -->
 
 * Returns: {string\[]}
@@ -1678,7 +1680,9 @@ const headerNames = response.getHeaderNames();
 ### `response.getHeaders()`
 
 <!-- YAML
-added: v7.7.0
+added:
+  - v8.0.0
+  - v7.7.0
 -->
 
 * Returns: {Object}
@@ -1705,7 +1709,9 @@ const headers = response.getHeaders();
 ### `response.hasHeader(name)`
 
 <!-- YAML
-added: v7.7.0
+added:
+  - v8.0.0
+  - v7.7.0
 -->
 
 * `name` {string}
@@ -2579,7 +2585,9 @@ exist in message, it will be `undefined`.
 ### `outgoingMessage.getHeaderNames()`
 
 <!-- YAML
-added: v8.0.0
+added:
+  - v8.0.0
+  - v7.7.0
 -->
 
 * Returns {string\[]}
@@ -2590,7 +2598,9 @@ names are lowercase.
 ### `outgoingMessage.getHeaders()`
 
 <!-- YAML
-added:  v8.0.0
+added:
+  - v8.0.0
+  - v7.7.0
 -->
 
 * Returns: {Object}
@@ -2617,7 +2627,9 @@ const headers = outgoingMessage.getHeaders();
 ### `outgoingMessage.hasHeader(name)`
 
 <!-- YAML
-added:  v8.0.0
+added:
+  - v8.0.0
+  - v7.7.0
 -->
 
 * `name` {string}