http: remove 'headers sent?' check in OutgoingMessage.getHeader()

Fixes #752.

lib/http.js

@@ -540,10 +540,6 @@ OutgoingMessage.prototype.getHeader = function(name) {
     throw new Error("`name` is required for getHeader().");
   }
 
-  if (this._header) {
-    throw new Error("Can't use mutable header APIs after sent.");
-  }
-
   if (!this._headers) return;
 
   var key = name.toLowerCase();

lib/http2.js

@@ -540,10 +540,6 @@ OutgoingMessage.prototype.getHeader = function(name) {
     throw new Error("`name` is required for getHeader().");
   }
 
-  if (this._header) {
-    throw new Error("Can't use mutable header APIs after sent.");
-  }
-
   if (!this._headers) return;
 
   var key = name.toLowerCase();