Spring Add ContentLength support for InputStreamResource created in ResourceHttpMessageConverter and ResourceDecoder

Issue InputStreamResource created in ResourceHttpMessageConverter /ResourceDecoder doesn't properly support contentLength. contentLength() method consumes input stream to calculate size instead of just providing predefined value.

Motivation Streaming use case, where application acts as a proxy for large amount of data.

Fix ResourceHttpMessageConverter should read HTTP Content-Length header. ResourceDecoder creates ISR based on byte array, so size is always known.

Caveats contentLength() for InputStreamResource created in ResourceHttpMessageConverter can be -1 if Content-Length header is missing.

Comment From: rstoyanchev

Thanks for the pull request. I made a small adjustment to fall back on the base class in case the content-length is not present.