:call-seq:
  get_encoding_from_headers(headers) -> String or nil

Return encoding from an HTTP header hash.

We use the RFC 7231 specification and do not impose a default encoding on
text. This differs from the older RFC 2616 behavior, which specifies
using ISO-8859-1 for text/* content types without a charset.

Strings will use the default encoding when this method returns nil. This
default is likely to be UTF-8 for Ruby >= 2.0

[Parameters:]

  - headers (Hash<Symbol, String>)

[Returns:]

  - (String, nil) -- Return the string encoding or nil if no header is found.

Examples:

  get_encoding_from_headers({:content_type => 'text/plain; charset=UTF-8'})
  => "UTF-8"