Issue #6988: expand_log(exp(x), force=True) = x

This is a try to fix issue #6988. If I made some mistake just let me know, as I'm still new to sympy development.

```
In [9]: expand_log(log(exp(x)), force=True)
Out[9]: x

In [10]: expand_log(log(y**(x)), force=True)
Out[10]: x⋅log(y)
```