Uses strings.EqualFold

[rkilingr]

Changes case insensitive string comparisons to string.EqualFold which performs better than strings.Lower(str) == str comparison

Already covered in test cases:
https://github.com/labstack/echo/blob/master/middleware/basic_auth_test.go#L49
https://github.com/labstack/echo/blob/master/context_test.go#L821

[codecov]

Codecov Report

Merging #1790 (5b0b25b) into master (b0f56ea) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1790   +/-   ##
=======================================
  Coverage   89.75%   89.75%           
=======================================
  Files          32       32           
  Lines        2675     2675           
=======================================
  Hits         2401     2401           
  Misses        175      175           
  Partials       99       99           

Impacted Files	Coverage Δ
context.go	87.19% <100.00%> (ø)
middleware/basic_auth.go	65.62% <100.00%> (ø)

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