Whitespace in URLs must be percent-encoded

Some ABNF construction rules for URL constituents allow whitespace, e.g. rules contributing to the $filter system query option. Currently these rules use the WSP sub-rule which allows the space and horizontal-tab character. These characters are not allowed in URLs and must be percent-encoded.

So we need to either

• redefine the whitespace rule to
  WSP = "%20" / "%09"

or

• define a rule for precent-encoded whitespace
  peWS = "%20" / "%09"
  and use it in URL-relevant rules.

The latter approach is more explicit and less misleading as the current definition of WSP is a widely used convention