Fixing issue with User-Agent getting overridden

closes #69

lib/adapters/http.js

@@ -26,7 +26,11 @@ module.exports = function httpAdapter(resolve, reject, config) {
   );
 
   // Set User-Agent (required by some servers)
-  headers['User-Agent'] = 'axios/' + pkg.version;
+  // Only set header if it hasn't been set in config
+  // See https://github.com/mzabriskie/axios/issues/69
+  if (!headers['User-Agent'] && !headers['user-agent']) {
+    headers['User-Agent'] = 'axios/' + pkg.version;
+  }
 
   if (data) {
     if (utils.isArrayBuffer(data)) {