Description:
------------
In some conditions, DateTime::diff makes a mistake on month-section.

For example, the gap of Feb 1 and Mar 1 is 1 month.
But output was "0 month 28 days" in timezone Asia/Tokyo.
("1 month 0 days" in timezone UTC)

Test script:
---------------
https://wandbox.fetus.jp/permlink/zgdq2iE1VuL9zM50

Expected result:
----------------
a: 2015-2-1 <--> 2015-3-1
DateInterval Object
(
    [y] => 0
    [m] => 1
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 28
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)


b: 2015-3-1 <--> 2015-3-29
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 28
    [h] => 0
    [i] => 0
    [s] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 28
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)


c: 2015-4-1 <--> 2015-4-29
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 28
    [h] => 0
    [i] => 0
    [s] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 28
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)

Actual result:
--------------
a(Asia/Tokyo): 2015-2-1 <--> 2015-3-1
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 28
    [h] => 0
    [i] => 0
    [s] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 28
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)


b(Asia/Tokyo): 2015-3-1 <--> 2015-3-29
DateInterval Object
(
    [y] => 0
    [m] => 1
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 28
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)


c(Asia/Tokyo): 2015-4-1 <--> 2015-4-29
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 28
    [h] => 0
    [i] => 0
    [s] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 28
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)