Add a test for downloading files.

There already was a test doing it, but it was rather concentrating on
authentication, so I renamed it to testFileDownloadAuth. The new test
checks how the server behaves when downloading nonexistent or files
with no read permissions.
Send a reply immediately if file is not readable to begin download.

Hinted by Steve Dabner.
Modify testFileDownload to expect specific errors.

File download handling is also modified to return either 404 or 403
when file is not found or couldn't be opened for reading. This commit
makes ServerTest::testGetShouldFail test failing.
Merge pull request #30 from krnowak/file-download-hang-fix

Improve error handling when downloading files (not found, or not readable). Fixes SOAP-77.