requests: nil pointer dereference panic was fixed [#25]

http.Do() method will return non-nil [error] in following cases:
1. Request is nil
2. Response missing Location header
3. Client failed to parse Location header
4. Method "request.GetBody()" returns error
5. Http.Client.Send() returns error
5. Client timeout was exceeded

Signed-off-by: Christoph Polcin <labs@polcin.de>

requests.go

@@ -38,6 +38,9 @@ func (c *Client) req(method, path string, body io.Reader, intercept func(*http.R
 	}
 
 	rs, err := c.c.Do(r)
+	if err != nil {
+		return nil, err
+	}
 
 	if rs.StatusCode == 401 && c.auth.Type() == "NoAuth" {
 		if strings.Index(rs.Header.Get("Www-Authenticate"), "Digest") > -1 {