Also adds test case which fails without this patch:
1) probeStream
should not fail when processing svg in multiple chunks:
Uncaught Error [ERR_STREAM_PUSH_AFTER_EOF]: stream.push() after EOF
2) probeStream
should not fail when processing svg in multiple chunks:
Error: done() called multiple times in test <probeStream should not fail when processing svg in multiple chunks> of file probe-image-size/test/test_stream.js
Fixes #52.
Also adds test case which fails without this patch: