This uses a negative lookahead assertion to ensure the colon is not followed by a character that will make it a key. In that case, it should be treated as a literal colon, and handled similar to other literal special characters.
WEBrick supports streaming responses since before it was imported into ruby 1.8.0, probably since it was written. If Sinatra doesn't support streaming with WEBrick it is a bug in Sinatra or rack and should be described that way explicitly. Thanks to @tenderlove for help with the README.ja.md. All others were checked in google translate.