Fixed issue 584

[Zhang-Muyang]

Fixed issue 584.
The problem is described in the issue. The reasons are as follows.

1. In original code, a special character may replace a digit, which leads to no digits.
2. The loop of special character may only iterates once, which generates no special character.
3. In the character function in Lorem, digits and letters are chosen randomly from array character which contains all letters and
   digits. But there could be chances that all character chosen are letters, no digits.
4. Similar to 3), there could be no upper letter in small chances.

The fix is:

1. Add a Test which is mainly copied from the issue description.
2. Overload the character function in Lorem. Its idea is: We choose the special, digit, upper letter first, which ensures we can defintely contain these. And then randomly choose the rest. Shuffle the characters array in the end.

Plus, feel free to contact with me if there is any problem. : )

[Zhang-Muyang]

Fix #584