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