Request interceptor headers

Currently it is possible to add a request interceptor and modify the config object for a http request before it is sent. I am able to add custom headers to the config object within an interceptor however these headers get ignored when the http request is constructed. I was able to fix this in my own fork by modifying the following line in the "serverRequest" function 
from:
```
var reqData = transformData(config.data, headersGetter(headers), config.transformRequest);
```
to:
```
var reqData = transformData(config.data, headersGetter(extend(headers,config.headers)), config.transformRequest);
```