Revert header collection behavior such that header values are always arrays of strings instead of only when duplicate headers are found

lib/imap.js

@@ -266,12 +266,10 @@ ImapConnection.prototype.connect = function(loginCb) {
 
             for (var i=0,k,len=info.headers.length; i<len; i+=2) {
               k = info.headers[i].toLowerCase();
-              if (headers[k] !== undefined) {
-                if (!Array.isArray(headers[k]))
-                  headers[k] = [headers[k]];
+              if (headers[k] !== undefined)
                 headers[k].push(info.headers[i + 1]);
-              } else
-                headers[k] = info.headers[i + 1];
+              else
+                headers[k] = [info.headers[i + 1]];
             }
 
             self._state.requests[0]._msg.headers = headers;