The source data should not be changed

[code position](https://github.com/RobinDavid/LSB-Steganography/blob/a1d4d8b70ad999d9d79b90d7a1c9ba8cb1976bf8/LSBSteg.py#L28)
add this:
```python
im=im.copy() #for ndarray,other Types also be like this
```
Otherwise he could change the source data and cause some trouble