fix trends not displayed correctly

[zimmer-yan]

PhpMetrics v2.9.1

Description:
Fixing trends not being displayed correctly.

Current behavior:
The trends in the html report will always show the current value and a trend of minus that value.

Correct behavior:
show actual trend change

Root cause:
Hal\Report\Html\Reporter.php:255 checks for isset($this->sum->$type->$key) which will never be set, resulting in $newValue being assigned 0, causing the trend to be 0 - $oldValue.

Fix:
Changing the lookup to $this->$type->$key from $this->sum->$type->$key checks for the correct variable.