В вопросе явно не указано, но речь, судя по всему, идет о типе application/x-www-form-urlencoded, а не multipart/form-data
Про него пишут вот что:
www.w3.org/TR/html401/interact/forms.html#h-17.13.4.1
Control names and values are escaped. Space characters are replaced by `+', and then reserved characters are escaped as described in [RFC1738], section 2.2: Non-alphanumeric characters are replaced by `%HH', a percent sign and two hexadecimal digits representing the ASCII code of the character. Line breaks are represented as "CR LF" pairs (i.e., `%0D%0A').
То есть да, пробелов в именах полей быть не должно.