coverage issue of blob file

[HemangChothani]

To reproduce run nox -s cover with the master branch
It's related to the PR #204

nox > pip install coverage pytest-cov
nox > coverage report --show-missing --fail-under=99 
Name                                   Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------------------------
google/cloud/__init__.py                   6      6      0      0     0%   15-22
google/cloud/storage/__init__.py           8      0      0      0   100%
google/cloud/storage/_helpers.py         106      0     27      0   100%
google/cloud/storage/_http.py             15      0      2      0   100%
google/cloud/storage/_signing.py         177      0     82      0   100%
google/cloud/storage/acl.py              172      0     40      0   100%
google/cloud/storage/batch.py            126      0     24      0   100%
google/cloud/storage/blob.py             517      0    180      1    99%   808->806
google/cloud/storage/bucket.py           558      0    174      0   100%
google/cloud/storage/client.py           207      0     68      0   100%
google/cloud/storage/constants.py         13      0      0      0   100%
google/cloud/storage/hmac_key.py          82      0     26      0   100%
google/cloud/storage/iam.py               23      0      0      0   100%
google/cloud/storage/notification.py     119      0     34      0   100%
tests/unit/__init__.py                     8      0      0      0   100%
tests/unit/test__helpers.py              338      0      0      0   100%
tests/unit/test__http.py                  45      0      0      0   100%
tests/unit/test__signing.py              494      0     58      0   100%
tests/unit/test_acl.py                   686      0      2      0   100%
tests/unit/test_batch.py                 389      0      6      0   100%
tests/unit/test_blob.py                 2552      0    156      0   100%
tests/unit/test_bucket.py               2263      0     52      0   100%
tests/unit/test_client.py                939      0     20      0   100%
tests/unit/test_hmac_key.py              292      0      6      0   100%
tests/unit/test_notification.py          307      0      2      0   100%
----------------------------------------------------------------------------------
TOTAL                                  10442      6    959      1    99%
nox > coverage erase
nox > Session cover was successful.