merge doesn't resolve SpecialTag directives

[yaythomas]

>>> ctx['a'] = True
>>> ctx.merge({'a': PyString('False')})
>>> print(ctx)
{'a': PyString('False')}

Expected is True.

in merge, amend:

# str not mergable, so it doesn't matter if it exists in dest
                if isinstance(v, str):
                    # just overwrite dest - str adds/edits indiscriminately
                    current[k] = self.get_formatted_string(v)

to

# str not mergable, so it doesn't matter if it exists in dest
                if isinstance(v, str):
                    # just overwrite dest - str adds/edits indiscriminately
                    current[k] = self.get_formatted_string(v)
                elif isinstance(obj, SpecialTagDirective):
                    new = obj.get_value(self)