Multiple calls to iter* fail with unhelpful error.

[Lukasa]

If you call the iter* methods more than once for a single streamed request, it fails with an extremely unhelpful error:

>>> r = requests.get('http://www.google.com/', stream=True)
>>> [x for x in r.iter_content(1024)]
>>> [x for x in r.iter_content(1024)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/requests/utils.py", line 341, in iter_slices
    while pos < len(string):
TypeError: object of type 'bool' has no len()

I think it's fine that you can't call the iter* methods twice on a streamed response, but we can easily catch this situation so we should.

[tijko]

I'm making a pull request with a RuntimeError being raised. What kind of exception would you ideally like to throw here?

[Lukasa]

I'm going to close this to centralise on #2241.