Validate charset before force_encoding

[kylekyle]

HTTP::Response::Body will force a response's contents to whatever encoding was specified in the Content-Type header. That encoding should be validated before applied since some common encodings don't appear in ruby's Encoding lookup table.

For example,

require 'sinatra'

get '/', provides: 'text/html; charset=utf8' do 
    "Hello!"
end

Pulling that page using HTTP ...

require 'http'

HTTP.get('http://localhost:4567').to_s

/Users/kyle/.rvm/gems/ruby-2.2.1/gems/http-1.0.2/lib/http/response/body.rb:40:in `force_encoding': unknown encoding name - utf8 (ArgumentError)
    from /Users/kyle/.rvm/gems/ruby-2.2.1/gems/http-1.0.2/lib/http/response/body.rb:40:in `to_s'
    from client.rb:3:in `<main>'