POC: Add symbol's assumptions to the global_assumptions context

```python
In [1]: from sympy.assumptions.assume import global_assumptions
In [2]: r = Symbol('r', real=True)
In [3]: x = Symbol('x')
In [4]: ask(Q.real(x))
In [5]: ask(Q.real(r))
Out[5]: True
In [6]: ask(Q.real(Symbol('spam')))
In [7]: ask(Q.real(Symbol('spam', real=True)))
Out[7]: True
In [8]: ask(Q.real(Symbol('spam')))
```

Aren't the keys already strings?