I agree that it should be specified what is considered "valid". For example RFC 3986 says that:

2.3.  Unreserved Characters

   Characters that are allowed in a URI but do not have a reserved
   purpose are called unreserved.  These include uppercase and lowercase
   letters, decimal digits, hyphen, period, underscore, and tilde.

      unreserved  = ALPHA / DIGIT / "-" / "." / "_" / "~"

However, according to the test cases (at least in python) the "~" should be escaped... but "/" shouldn't be escaped!

must return valid link
which means all special characters should be escaped as usual