Error when trying to read 0 byte from StreamingBody

[youhyunkim]

Referring to the read method of StreamingBody:

botocore/botocore/response.py

Lines 69 to 81 in c632931

	def read(self, amt=None):
	"""Read at most amt bytes from the stream.
	If the amt argument is omitted, read all data.
	"""
	chunk = self._raw_stream.read(amt)
	self._amount_read += len(chunk)
	if not chunk or amt is None:
	# If the server sends empty contents or
	# we ask to read all of the contents, then we know
	# we need to verify the content length.
	self._verify_content_length()
	return chunk

If anyone asks for 0 bytes from a StreamingBody, the conditional on line 76 will pass because chunk is empty (since 0 bytes were asked for) and amount was set to 0 (not None). This leads to the content length verification, which will fail because you've read 0 bytes so far out of the entire content.

Might be an odd use case, but I feel like is a valid use case.
In fact, I ran into this issue when trying to use the ijson package link.
That library uses .read(0) in order to figure out what type of encoding the stream reader should use. Whether that's the best way to do it or not, I'm not entirely sure. But I feel like .read(0) should still be supported.

If you guys agree that it should be supported, maybe considering a condition like this:

if (not chunk and amt > 0) or amt is None:

[joguSD]

Thanks for the PR @youhyunkim. I've cleaned up the PR to pass all unit tests and added a test to validate this change in #1312.