Update Helper.php

[Axent96]

get Body return StreamInterface & not string

[judgej]

The reason for casting to a string, is that the whole body is streamed and serialised, beginning to end. Using getContents() will only stream what remains of the body, which may be nothing if another step has already read it, so calling extractMessageData() twice will only return the body the first time, and nothing the second time. Casting to string avoids this, and is documented in the PSR-7 recommendation.

Was there a reason for changing that that I'm perhaps missing?