absurlreplacer: write replacements directly to the byte buffer

absurlreplacer: write replacements directly to the byte buffer

The extra step isn't needed and this makes the code simpler.

And slightly faster:

benchmark              old ns/op     new ns/op     delta
BenchmarkAbsUrl        19987         17498         -12.45%
BenchmarkXmlAbsUrl     10606         9503          -10.40%

benchmark              old allocs     new allocs     delta
BenchmarkAbsUrl        28             24             -14.29%
BenchmarkXmlAbsUrl     14             12             -14.29%

benchmark              old bytes     new bytes     delta
BenchmarkAbsUrl        3512          3297          -6.12%
BenchmarkXmlAbsUrl     2059          1963          -4.66%