Test case insensitive header access
Ensure all headers can be accessed case insensitive

- Fix case insensitive header access via `getResponseHeader`.
- Refactor `getAllResponseHeaders` and `setRequestHeader` to be compatible.

Fixes #16
Update license copyright year
Add @gasi as author
Bump version
Add missing newline
Simplify expression
Merge pull request #17 from FiftyThree/case-insensitive-headers

Support case insensitive header access via `getResponseHeader`