Description:
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`DateTime::createFromFormat($format, $time, DateTimeZone $timezone = null)` has a default value for its third parameter `$timezone`, but it doesn't accept `null` as a value.

Running the following script will raise a warning:

DateTime::createFromFormat('Y/m/d', '2015/1/1', null);

// Warning: DateTime::createFromFormat() expects parameter 3 to be DateTimeZone, null given

While the documentation states that the default timezone is going to be used when no parameter is given, also the case where a `null` `$timezone` is given should be handled correctly.

This would align the API to the language semantics about default values.

Test script:
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<?php

var_dump(get_class(DateTime::createFromFormat('Y/m/d', '2015/1/1', null)));

Expected result:
----------------
string(8) "DateTime"

Actual result:
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Warning: DateTime::createFromFormat() expects parameter 3 to be DateTimeZone, null given in %a on line 3
Warning: get_class() expects parameter 1 to be object, boolean given in %a on line 3 bool(false)