use passed request size if content length is not provided

[cavaliercoder]

Hey thanks for submitting this. I'm not sure I understand the problem this solves though. Can you please elaborate on the issue you are solving with this PR?

The purpose of this function is to test:

• If the user expects a certain size (Request.Size)
• And the remote server advertises a known size (Response.HTTPResponse.ContentLength) for the requested range
• Does the size of any bytes resumed from disk, plus the remaining bytes on the remote server equal the size the user expects.

At all times, Response.Size should represent the file size that grab thinks is true, given the state of the world - not what the user expects it to be.

[cavaliercoder]

Closing due to inactivity. Please reopen and respond to the above questions if this is still important.