ElementProxy presence checks should invalidate cache

[dtognazzini]

See #123 (comment) for discussion.

When checking for presence, the DOM is not queried if the element was cached due to a previous lookup:

ae_page_objects/lib/ae_page_objects/element_proxy.rb

Line 142 in 135ddad

@loaded_element ||= load_element

Since #65, this is not true for absence checks. With absence checks, the cached element is ignored and the DOM is queried:

ae_page_objects/lib/ae_page_objects/element_proxy.rb

Line 146 in 135ddad

load_element

Both types of checks should invalidate the cache by looking up the element in the DOM.

[dtognazzini]

master: #158
10stable: #160