views_json: HTML entities (such as ') still appear even with $plaintext_output chosen

in views_views_json_style.theme.inc, use html_entity_decode() when the option for plaintext_output = TRUE and "field_output" = normal.
This way, HTML entities that come from field values are made into true plain text.
We also must ensure that the field output is UTF-8, in order to be compliant with JSON.

$content = $plaintext_output ? strip_tags($field->content) : $field->content;

Becomes:

$content = $plaintext_output ? strip_tags(html_entity_decode($field->content, ENT_QUOTES)) : $field->content;
$content = mb_check_encoding($content, 'UTF-8') ? $content : utf8_encode($content);

This functionality is already in place for view_xml plugin. See /views/theme/views_view.xml_style.theme.inc

We use the ENT_QUOTES option to convert single and double quotes (') and ("") as those are commonly encoded entities by Drupal Core.
Example is with Node titles. As of Drupal 7, all node titles are run through htmlentities(), so characters like apostrophes become ', when in reality we want an actual apostrophe in our plaintext output.

Patch below.