[MERGE] Fix test suite regression on OSX in TestReadMergeableFromUrl.test_smart_server_connection_reset.

[John Arbash Meinel]

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>> I've been bitten by that in another project (I used 'is' here by
>> copy/paste, I should have known better :) and I was wondering:
>> using 'is' seems to be an optimized path when both strings are
>> interned, but '==' should knows that and use it, so is it really
>> worth trying to use 'is' ? And if yes, in what contexts ?
> 
> “is” is useful in two cases that I can think of:
> 
>   * when you really do want to know if one object is the same instance
>     as another.  Use cases are things like implementing object
>     serialisers like pickle, or maybe caching/memoising mechanisms, or
>     debug tools.
> 
>   * when being as fast as possible is important, and you are *sure* that
>     “is” will give the correct answer in this case.
> 
> -Andrew.

The big one here is "is None". Occasionally you'll have a sentinel
object like:

_sentinel = object()

def foo():
  if X is true:
    return _sentinel


and then the caller can do

result = foo()
if result is _sentinel:
  # something special happened

John
=:->





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