Take the array key into account when matching up FOREIGN KEYs in DatabaseTableChangeP...

Take the array key into account when matching up FOREIGN KEYs in DatabaseTableChangeProcessor

Previously a FOREIGN KEY within the database that looks like this:

    […] FOREIGN KEY (someOtherUserID) REFERENCES wcf1_user (userID) […]

would match up a FOREIGN KEY definition like the following:

    […] FOREIGN KEY (userID) REFERENCES wcf1_user (userID) […]

Converted into the `getDiffData()` representation of the PHP DDL API these
would like:

    [ 'columns'           => 'someOtherUserID'
    , 'referencedColumns' => 'userID'
    , 'referencedTable'   => 'wcf1_user'
    ]

and

    [ 'columns'           => 'userID'
    , 'referencedColumns' => 'userID'
    , 'referencedTable'   => 'wcf1_user'
    ]

respectively.

Now taking the diff of the second array against the first array (subtracting
the first from the second) will remove *both* 'userID' values, resulting in an
empty difference, thus believing both FOREIGN KEYs are identical

Fix this issue by using `array_diff_assoc` which will also take the key into
account.