beego/context: Fix ignored Header in case SetStatus has been called before

### Problem description
By calling this.Ctx.Output.SetStatus(xyz) the Response Header is sent immediately. Headers that are added afterwards are ignored, including the Content-Encoding (e.g. gzip) and the Content-Type (e.g. Json).
This prevents us from serving Json in body with an error messages together with the http-code.

### Fix
I removed the call to WriteHeader in SetStatus, so the header is not sent when SetStatus is called, instead it is postponed  two places:
1. context's function "Body", right before writing the body (which implicitly writes the header first, but with status code 200 by default)
2. at the end of beego's ServeHTTP-method in router.go, but only if the status has been changed (if it is not 0)
The first case only happens when controller's ServeXYZ is called. By setting output.Status to 0 I prevent WriteHeader from being executed twice.

### Example of header before/after fix
```
func (this *SomeController) Get() {
	// ...
	responseBody := ErrorResponse{Message: "AccessToken invalid"}
	this.Data["json"] = responseBody
	this.Ctx.Output.SetStatus(403)
	this.Ctx.Output.Header("Access-Control-Allow-Origin", "*")
	this.ServeJson()
}
```
Resulted before in this response header:
```
Status 403 Forbidden
Date: Tue, 08 Jul 2014 20:37:23 GMT 
Content-Length: 61 
Content-Type: application/x-gzip
```
With gzip the body cannot be automatically decompressed due to missing header "Content-Encoding"

Results now in this response header:
```
Access-Control-Allow-Origin: *
Content-Encoding: gzip 
Content-Type: application/json;charset=UTF-8 
Server: beegoServer:1.3.2 
Date: Tue, 08 Jul 2014 20:39:34 GMT 
Content-Length: 61
```