Add a method to send post data with a PUT method type

You can now use setPostText(String postText, Method method) to use
post data text with the PUT method instead of POST method.
The setPostText(String postText) is still use the POST method by default
for compatibility.

The code in NetworkConnectionImpl was already ready for using post data
with the PUT method
Prevent a NPE when the request contains a parameter with a null value

If one of the parameters from a request had a null value, the execution
was crashing with a NullPointerException.
The null value is now replaced by an empty string.

Also fix some logs

Fixes #30
Merged with upstream