Encode a string to the "x-www-form-urlencoded" form, enhanced
with the UTF-8-in-URL proposal. This is what happens:
The ASCII characters a-z, A-Z, and 0-9 remain the same.
The unreserved characters - _ . ! ~ * ' ( ) remain the same.
The space character ' ' is converted into a plus sign '+'.
All other ASCII characters are converted into the 3-character string "%xy", where xy is the two-digit hexadecimal representation of the character code.
All non-ASCII characters are encoded in two steps:
first to a sequence of 2 or 3 bytes, using the UTF-8 algorithm;
secondly each of these bytes is encoded as "%xx".
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