Sonar: Reduce cyclomatic complexity

Reduce the number of conditional operators for equals(). Improve
testEquals() to better cover this method.

This patch also:

* immediately returns expression instead of assigning it to the
  temporary variable "str",
* adds the "@Override" annotation above equals() method signature.

Issue-ID: PORTAL-595
Change-Id: I15f600acce873eb3f22cc405d06a50890c7e87c3
Signed-off-by: Dominik Mizyn <d.mizyn@samsung.com>