buildCanonicalHeaders should lowercase header names before sorting

[eranation]

https://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html

To create the canonical headers list, convert all header names to lowercase and remove leading spaces and trailing spaces. Convert sequential spaces in the header value to a single space.

I think that right now it sorts first, lower case later.

[kndt84]

Thansk for sendig a PR