Use urlencode instead of esc_attr for search terms

[jmdodd]

Spaces between terms are getting lost with current esc_attr.

[johnjamesjacoby]

Still worth using esc_attr() in the input box (I think) but the URL should definitely get the urlencode() treatment.

[jmdodd]

(In [4581]) Replace esc_attr() with urlencode() when adding search terms query args.

• Fixes #2110.

[jmdodd]

form-search.php uses esc_attr(); this was specifically for dealing with the add_args situations like pagination.