Buffer only proxy header data

[AlexanderYastrebov]

Reverts and re-implements incorrect #116
which passed all connection data through the buffered reader.

[pires]

@mmatczuk care to take a look at this?

[coveralls]

coverage: 94.67% (+0.5%) from 94.136%
when pulling 0067a88 on AlexanderYastrebov:no-buffer
into bac82fd on pires:main.

[mmatczuk]

I added my comments in #116 (comment).

[mmatczuk]

Sent a followup patch #120.

[mmatczuk]

@AlexanderYastrebov

It also has issues.

PTAL at my comments in #120.

Besides I fixed WriteTo instead of hand-rolled reads.

Sure, this is also fixed in #120

BTW adding the WriteTo to the interface if the underlying connection does not implement in may trick io.Copy with buffer from a buffer pool to falling back to plain io.Copy that allocates. Also the WriteTo only works on Linux IF the writer is UnixConn.

Cheers