After sniffing and buffering data, if we try to read from
the socket again, bufio.Reader may block. This breaks HTTP handlers
in go1.5.2+ if one tries on browsers or with curl. Go's HTTP client,
however, is not broken. This issue is also there with TeeReader.
Return immediately with the data in the sniffed buffer.
bufferedReader is an optimized implementation of io.Reader that behaves
like
```
io.MultiReader(bytes.NewReader(buffer.Bytes()), io.TeeReader(source, buffer))
```
without allocating.
This has a measurable effect on benchmarks:
```
name old time/op new time/op delta
CMuxConn-4 1.09µs ± 4% 0.99µs ±19% -9.32% (p=0.000 n=17+19)
name old alloc/op new alloc/op delta
CMuxConn-4 240B ± 0% 260B ± 0% +8.33% (p=0.000 n=20+20)
name old allocs/op new allocs/op delta
CMuxConn-4 9.00 ± 0% 5.00 ± 0% -44.44% (p=0.000 n=20+20)
```
Note that appropriate test coverage is provided by `TestRead`.
