RF convenience methods for handling booleans, example

| Some Keyword | False |

Is evaluated as False in Robot Framework standard libraries, although the argument is string and according Python it should evaluate as True. Changing the S2L to behave in similar fashion as the Robot Framework standard libraries could be useful.

More details in http://robotframework.org/robotframework/latest/libraries/BuiltIn.html#Boolean%20arguments and some discussion in #717