Cors preflight support

[ckkz-it]

#97

[hzlmn]

Hi, thanks for PR. Seems like some problems with GitHub travis ci status update, but could You fix linting error
0.36s$ flake8 --show-source tests tests/test_middleware.py:249:80: E501 line too long (95 > 79 characters) async def test_pass_preflight_options_request(create_app, aiohttp_client, fake_payload, token):
other from these small fixes, everything LGTM.

[codecov]

Codecov Report

Merging #139 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
+ Coverage   98.13%   98.16%   +0.03%     
==========================================
  Files           4        4              
  Lines         107      109       +2     
==========================================
+ Hits          105      107       +2     
  Misses          2        2              

Impacted Files	Coverage Δ
aiohttp_jwt/middleware.py	100.00% <100.00%> (ø)

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[hzlmn]

Thanks.