Max upload size 0 when post_max_size = 0

[moscar09]

As a convention, post_max_size can be set to 0 to disable any limitation on max post size.

Quote from php.ini:

; Maximum size of POST data that PHP will accept.
; Its value may be 0 to disable the limit. It is ignored if POST data reading
; is disabled through enable_post_data_reading.
; http://php.net/post-max-size

WordPress does not take this into account in wp-admin/includes/template.php

[SergeyBiryukov]

#26918 was marked as a duplicate.

[SergeyBiryukov]

It's worth noting that memory_limit can also affect file uploading:
​http://php.net/manual/en/ini.core.php#ini.post-max-size

[SergeyBiryukov]

#24558 was marked as a duplicate.

[biskobe]

Properly determine the file size if one of the values, checked in the function is zero, meaning unlimited.

[biskobe]

Tests for modified wp_max_upload_size function in previous patch.

[biskobe]

Updated documentation for the change.

[biskobe]

Added proper check for the optional parameters values. It would've failed with zero inputs.

[biskobe]

Previous tests didn't catch the problem because the values that I tested with were too small