A self link should only require one allocation, and we should skip
url.PathEscape() except when the path actually needs it.

Add a fuzz test to build random strings and verify them against
the optimized implementation. Add a new BenchmarkWatchHTTP_UTF8 that
covers when we have unicode names in the self link.

```
> before
BenchmarkGet-12          	   10000	    118863 ns/op	   17482 B/op	     130 allocs/op
BenchmarkWatchHTTP-12    	   30000	     38346 ns/op	    1893 B/op	      29 allocs/op

> after
BenchmarkGet-12               	   10000	    116218 ns/op	   17456 B/op	     130 allocs/op
BenchmarkWatchHTTP-12         	   50000	     35988 ns/op	    1571 B/op	      26 allocs/op
BenchmarkWatchHTTP_UTF8-12    	   50000	     41467 ns/op	    1928 B/op	      28 allocs/op
```

Saves 3 allocations in the fast path and 1 in the slow path (the
slow path has to build the buffer and then call url.EscapedPath
which always allocates).