Assigning segments_ref to segments_ref

Consider the following:
```
url u1( "/index%2Ehtm" );
url u2( "/path/to/file%2Etxt" );
segments_ref ps1 = u1.segments();
segments_ref Type ps2 = u2.segments();
ps1 = p2;
```

What should the contents of ps1 be? Well, `operator=` is documented with this effect:
```
ps1.assign( ps2.begin(), p2.end() );
```

Iterating ps2 will produce decoded segments, so ps1 will end up with
```
{ "path", "to", "file.txt" }
```

Note how "file.txt" has the escape removed. This behavior is technically correct according to our specification but is this the right thing to do?