Normalize the URL fragment
Applies Syntax-based normalization to the URL fragment.
Percent-encoding triplets are normalized to uppercase letters. Percent-encoded octets that correspond to unreserved characters are decoded.
assert( url( "http://www.example.com#%61bc" ).normalize_fragment().buffer() == "http://www.example.com#abc" );
Strong guarantee. Calls to allocate may throw.
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