rfc3986
2.1.  Percent-Encoding
> "%20" is the percent-encoding for the binary octet
   "00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
   character (SP).
rfc1866
8.2.1. The form-urlencoded Media Type
1. The form field names and values are escaped: space
        characters are replaced by `+', and then reserved characters
cf. IPA URI のエスケープ