Fixed pos/neg assumption for arc trigonometric functions

Trying to fix https://github.com/sympy/sympy/issues/7246

Though I'm facing some problem.
```
>>> from sympy import *
>>> p = Symbol('p', positive=True)
>>> f = atan(p)
>>> f.is_positive
True  #Expected
>>> f.is_negative
True  #Expected
>>> u = Symbol('u')
>>> mf = atan(u)
>>> mf.is_positive
True  #Not Expected
```

But if I do only the second part it works well

```
>>> from sympy import *
>>> u = Symbol('u')
>>> mf = atan(u)
>>> mf.is_positive
>>> #Expected
```

Maybe ```cls.is_positive``` or ```cls.is_negative``` is not the correct way to do it. Please help.