'Replace some strings by it\'s hex encoded value.
:return: a string.'

def _mutate(self, data):

    new_data = ''
    for char in data:
        if (char not in ['?', '/', '&', '\\', '=', '%', '+']):
            if (randint(1, 2) == 2):
                char = ('%%%02x' % ord(char))
        new_data += char
    return new_data
