FileBasedWebSessionCacheTest: Fix IncompatibleArgumentType error

Compilation of the tests under bazel fails with:

  error: [IncompatibleArgumentType]
    Argument 'path' should not be passed to this method.
    Its type Path is not compatible with the required type: String.
      assertThat(cache.getIfPresent(path)).isNull();
                                    ^
      (see http://errorprone.info/bugpattern/IncompatibleArgumentType)

Change-Id: I97ca1f2242ffe0d1698ccb0c2c792fdb12169d3d
diff --git a/src/test/java/com/ericsson/gerrit/plugins/highavailability/websession/file/FileBasedWebSessionCacheTest.java b/src/test/java/com/ericsson/gerrit/plugins/highavailability/websession/file/FileBasedWebSessionCacheTest.java
index 7089b7b..9b911c4 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/highavailability/websession/file/FileBasedWebSessionCacheTest.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/highavailability/websession/file/FileBasedWebSessionCacheTest.java
@@ -130,7 +130,7 @@
   public void getIfPresentInvalidKeyTest() throws Exception {
     loadKeyToCacheDir(INVALID_KEY);
     Path path = websessionDir.resolve(INVALID_KEY);
-    assertThat(cache.getIfPresent(path)).isNull();
+    assertThat(cache.getIfPresent((Object) path)).isNull();
   }
 
   @Test