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.

* methods/el-get-http.el
(el-get-http-checksums): remove.
(el-get-http-retrieve-callback): don't compute hash on HTTP headers.
(el-get-http-compute-checksum): always compute a fresh hash.

* test/el-get-issue-1752.el: new test.