Fix #865: Don't strip invisible text when formatting hover string

This was introduced in #482 due to a bad interaction with a specific
server.  But this solution makes hyperlinks in Eldoc buffers
unclickable, because the markdown-mode function that visits a link
relies on the invisible text.

Per #866 

* eglot.el (eglot--format-markup): Use buffer-string instead of 
filter-buffer-substring
#865: joaotavora/eglot#865
#482: joaotavora/eglot#482
#866: joaotavora/eglot#866

lisp/progmodes/eglot.el

@@ -1436,7 +1436,7 @@ Doubles as an indicator of snippet support."
 	    (message-log-max nil))
         (ignore-errors (delay-mode-hooks (funcall mode))))
       (font-lock-ensure)
-      (string-trim (filter-buffer-substring (point-min) (point-max))))))
+      (string-trim (buffer-string)))))
 
 (define-obsolete-variable-alias 'eglot-ignored-server-capabilites
   'eglot-ignored-server-capabilities "1.8")