fixed SimpleStreamResponseSender ignored defined content length

[weierophinney]

Provide a narrative description of what you are trying to accomplish:

We are using the stream HTTP response to directly stream file content from S3 to clients.
As we are dealing with video files we are using the the Range/Content-Range headers to allow clients to download specific parts of files.

To be able to do so we use the Zend\Http\Response\Stream::setContentLength method together with fseek to define the parts of the file to send.

Where Stream::readStream is dealing with the content length it's not handled in SimpleStreamResponseSender and the stream gets send until EOF.

To fix that this PR uses stream_copy_to_stream to php://output instead of fpassthru if a content length has been defined.

• Are you fixing a bug?

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Originally posted by @marc-mabe at zendframework/zend-mvc#315