Fix some minor gravatar.el issues

Fix some minor gravatar.el issues

For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00528.html
* lisp/image/gravatar.el (gravatar-hash): Trim leading and trailing
whitespace in given address, as per the Gravatar docs.
(gravatar-retrieve-synchronously): Silence call to
url-retrieve-synchronously for consistency with gravatar-retrieve.
(gravatar-retrieved): Only cache buffer on successful retrieval.
* test/lisp/image/gravatar-tests.el: New file.