Don't cache http checksums.

- If the file was changed on the HTTP server before you restarted emacs, the
  checksum wouldn't be recomputed because it would be taken from the
  cache.

- The cached value was wrongly including the HTTP headers in the
  computation. Added a test for this.

- Computing a SHA1 is not so expensive that it needs the complication of
  caching.

fixes #1752