[#2629] Add charset to Content-Type header

- Defaults to UTF-8
- Allows setting via a setter
- charset is always used in generated content-type header
Incorporate feedbac

- Added getter for charset
- For selected multibyte character sets, an additional
  content-transfer-encoding header is set with the value 'BINARY'
Merge pull request #3483 from weierophinney/feature/json-strategy-charset

[#2629] Add charset to Content-Type header