canonifyurls in srcset

canonifyurls in srcset

Speed is about the same as before, uses slightly less memory:

```
benchmark              old ns/op     new ns/op     delta
BenchmarkAbsURL        17302         17713         +2.38%
BenchmarkXMLAbsURL     9463          9470          +0.07%

benchmark              old allocs     new allocs     delta
BenchmarkAbsURL        28             24             -14.29%
BenchmarkXMLAbsURL     14             12             -14.29%

benchmark              old bytes     new bytes     delta
BenchmarkAbsURL        3422          3144          -8.12%
BenchmarkXMLAbsURL     1985          1864          -6.10%
```

Fixes #1059