Scheme names consist of a sequence of characters beginning with a letter and followed by any combination of letters, digits, plus ("+"), period ("."), or hyphen ("-"). RFC 2396 有下面的一段
The plus "+", dollar "$", and comma "," characters have been added to those in the "reserved" set, since they are treated as reserved within the query component. 表示加号已经是url的保留字了，不需要转义。
然后html4文档里才有关于加号的转义：
application/x-www-form-urlencoded Forms submitted with this content type must be encoded as follows: Control names and values are escaped. Space characters are replaced by`+', and then reserved characters..... 声明只有content-type为application/x-www-form-urlencoded时才会对+做转义。
