[BUGFIX] Empty JSON-Content will be added to request

If you create an POST-Request (e.g. with an forking action) the HTTPClient
will be created with a post content "[]".
This can create some problems on requesting path.

Solution: Check if the content is empty.
Merge pull request #33 from andygrunwald/empty-content-in-request

[BUGFIX] Empty JSON-Content will be added to request