Set if the path is absolute
This function adjusts the path to make it absolute or not, depending on the parameter.
If an authority is present, the path is always absolute. In this case, the function has no effect.
url u( "path/to/file.txt" );
assert( u.set_path_absolute( true ) );
assert( u.buffer() == "/path/to/file.txt" );
this->is_path_absolute() == true && this->encoded_path().front() == '/'
path = path-abempty ; begins with "/" or is empty
/ path-absolute ; begins with "/" but not "//"
/ path-noscheme ; begins with a non-colon segment
/ path-rootless ; begins with a segment
/ path-empty ; zero characters
path-abempty = *( "/" segment )
path-absolute = "/" [ segment-nz *( "/" segment ) ]
path-noscheme = segment-nz-nc *( "/" segment )
path-rootless = segment-nz *( "/" segment )
path-empty = 0
true on success.