fixes header case and brings in line with botocore

[thehesiod]

this fixes #131 by using the same case insensitive dict that botocore uses.

[thehesiod]

I'm merging ignoring the delta coverage for endpoint.py since I just moved code. This code is copied from botocore and we never had tests for them before.

[codecov-io]

Codecov Report

Merging #132 into master will increase coverage by 3.31%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
+ Coverage   81.63%   84.94%   +3.31%     
==========================================
  Files           7        7              
  Lines         392      392              
==========================================
+ Hits          320      333      +13     
+ Misses         72       59      -13

Impacted Files	Coverage Δ
aiobotocore/config.py	100% <100%> (+41.17%)	✅
aiobotocore/endpoint.py	84.32% <55.55%> (-0.75%)	❌

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