Unshadow list built-in in httputil.HTTPHeaders.get_all()

The HTTPHeaders method code:
```python
def get_all():
    for name, list in self._as_list.items():  # or even self._as_list.iteritems()
...
```
may be produced by 2to3 into:
```python
def get_all():
    for name, list in list(self._as_list.items()):
...
```
which leads to error due to shadowing of "list' built-in by local variable named "list".
And I include docstrings quoting fixes in the pull request.