Allow space in-between checksums in response headers

[mcsimps2]

Summary: I encountered this bug when downloading files using the Google Cloud Storage Python libraries, but the bug is coming from the Resumable Media library. The current code (and official docs) assumes the checksum headers will be contiguous without whitespace:

'x-goog-hash': 'crc32c=t6zgHw==,md5=zH3E9XwJPGGra4vnT+aamQ=='

However, I'm actually getting responses with whitespaces between each type of checksum:

'x-goog-hash': 'crc32c=t6zgHw==, md5=zH3E9XwJPGGra4vnT+aamQ=='

Because of this, the checksum of downloaded files aren't being verified.

Solution is to patch _helpers.py as follows:

def _parse_checksum_header(header_value, response, checksum_label):
    if header_value is None:
        return None

    matches = []
    for checksum in header_value.split(u","):
        name, value = checksum.split(u"=", 1)
        # Use lstrip to support both "," and ", " as delimiters
        if name.lstrip() == checksum_label:
            matches.append(value)

    if len(matches) == 0:
        return None
    elif len(matches) == 1:
        return matches[0]
    else:
        raise common.InvalidResponse(
            response,
            u"X-Goog-Hash header had multiple ``{}`` values.".format(checksum_label),
            header_value,
            matches,
        )

Relevant: #22, #31, #32