fix($http) - run execHeaders on merged header object

fixes angular#5784

Author: perek

src/ng/http.js

@@ -766,9 +766,8 @@ function $HttpProvider() {
 
         defHeaders = extend({}, defHeaders.common, defHeaders[lowercase(config.method)]);
 
-        // execute if header value is function
+        // execute if header value is a function for defaults
         execHeaders(defHeaders);
-        execHeaders(reqHeaders);
 
         // using for-in instead of forEach to avoid unecessary iteration after header has been found
         defaultHeadersIteration:
@@ -784,6 +783,8 @@ function $HttpProvider() {
           reqHeaders[defHeaderName] = defHeaders[defHeaderName];
         }
 
+        // execute if header value is a function for merged headers
+        execHeaders(reqHeaders);
         return reqHeaders;
 
         function execHeaders(headers) {