ReadOnlySubStream shold not throw NotSupportedException on getting Length

[AlexeiScherbakov]

Even for archives without any compression ReadOnlySubStream throws exception on getting length
I think it should return initial value of bytesToRead getted in constructor

[jcapellman]

Agreed - just hit this

[adamhathcock]

Not sure I agree. The intention is that streams are forward only. Header information would have length if you needed it.

Willing to look at a PR

[jcapellman]

The use case for me was upon inspecting archives within an archive to determine what file type it was using a cigam check (i.e. reading the first couple bytes) I was attempting to set position to 0 afterwards so I could do further processing of the stream.

[adamhathcock]

I do the same in ReaderFactory with RewindableStream which is built for this use case on forward only streams: https://github.com/adamhathcock/sharpcompress/blob/master/src/SharpCompress/Readers/ReaderFactory.cs