Added BASE64 to set screenshot directory keyword

Forgot to add in the setting of the screenshot directory to handle BASE64 when not in all caps.

Author: emanlove

src/SeleniumLibrary/keywords/screenshot.py

@@ -61,6 +61,8 @@ def set_screenshot_directory(self, path: Union[None, str]) -> str:
             path = None
         elif path.upper() == EMBED:
             path = EMBED
+        elif path.upper() == BASE64:
+            path = BASE64
         else:
             path = os.path.abspath(path)
             self._create_directory(path)