__toString() always return empty content when resource is not seekable

[svycka]

I have noticed this with amazon S3 it does return resource with meta seekable => false and I can't use this I have to create file and the n create new stream from it. Also does not make sense to rewind not seekable stream.

[svycka]

what's strange here that hhvm did not fail here: https://travis-ci.org/zendframework/zend-diactoros/builds/223063138
@weierophinney also not sure but maybe we should add this check also for phpStream and RelativeStream?

[weierophinney]

@svycka The PhpInputStream is always seekable, and we have functionality to ensure rewinding works (by caching the data as we read). The RelativeStream implementation, since it delegates to another stream, likely needs the change as well; I'll try and write a test as I merge this to see what happens.

[weierophinney]

Thanks, @svycka