add test for StreamSocketClient#read
add StreamSocketClient#contents to get the remaining content of a stream
Rely on StreamSocketClient#contents to read to complete response

Reading the written number of bytes worked in this case, since the request was
always larger than the response from Graylog. right now, the only thing we check
is that the returning status-code matches our expectation - having the complete
response might be helpful to debug things in the future.
add test for http-transport
add separate test for AbstractTransport#publish