* upstream/master: (28 commits)
  Separate collections from collections.abc
  Remove trailing comma
  Make header parsing more RFC-7230-compliant
  raising the signleton NotImplemented actually raises a TypeError and not NotImplementedError - This issue can cause wrong exception handling or even no handling
  Pass python_requires argument to setuptools
  Add test for ConnectionPool.close
  Prevent AttributeError to be raised when pool is already None
  ;;
  Add pyenv install for Python 3.7 on macOS
  Add macOS+Python 3.7 to Travis
  Add py37 to tox.ini envlist
  Begin testing against Python 3.7
  Prefer readthedocs.io instead of readthedocs.org for doc links
  Move RECENT_DATE to 2017-06-30
  Add spacing between sections on README
  Add Akamai as a sponsor; update sponsorship text to include time-based contributions
  Handle gzip responses with multiple members
  Use FQDN only for DNS and drop trailing dot for other operations
  Credit HPE for my work
  Change use of deprecated assertEquals to pytest style assert
  ...

Signed-off-by: Isaac To <isaacto@gmail.com>

AWS S3 servers set "Transfer-Encoding: chunked" even for a HEAD
response, which results in urllib3 not returning the connection to the
pool after calling stream() or read_chunked().

The patch moves the HEAD request check into the error_catcher context
manager, which ensures to release the connection to the pool upon
completion.

Fixes #1234