Confirm buffer is not empty before erasing

Confirm file_exists before opening. For PHP 5.3 or earlier, check if
safe_mode is active to avoid errors.
Use wp_send_json() WordPress function for AJAX

The header(), json_encode(), and die(), functions are all executed with
a better logical routine that should provide consistent results.
Make checkout form multipart
Allow downloadable products URL to be relative
Remove ordering query args

If these are not removed right after use they will affect the remaining
queries on the page.
status report overrides table format

New tr for every extension being checked for overrides.
Update class-wc-query.php

Use 'term_id' instead of 'id' for the tax_query as described in WP codex: http://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters
Merge remote-tracking branch 'upstream/master'
Introduce 'woocommerce_get_order_item_totals_excl_free_fees' filter

The new filter will allow plugins that add order item meta to choose to
display the free fee items on order details pages and emails.
order item count wrong
filter output apart from count
filter name change
filter class
add filter to is_purchasable()
Update woocommerce.php
Update class-wc-template-loader.php
Field show/hide options for stock fields and variations
add filter to method get_sku()
Product admin/saving
Update copy-pasted docs

Fixes #5707