Fix --splinter-headless, as it could as well be of type bool, not str.

pytest_splinter/plugin.py

@@ -237,7 +237,7 @@ def splinter_screenshot_dir(request):
 @pytest.fixture(scope="session")
 def splinter_headless(request):
     """Flag to start the browser in headless mode."""
-    return request.config.option.splinter_headless == True
+    return request.config.option.splinter_headless
 
 
 @pytest.fixture(scope="session")  # pragma: no cover
@@ -781,5 +781,4 @@ def pytest_addoption(parser):  # pragma: no cover
         help="Run the browser in headless mode.",
         action="store_true",
         dest="splinter_headless",
-        default="false",
     )