http.ServeContent reads too much before calling DetectContentType

[tbushnell]

The documented interface for http.DetectContentType says that it only uses 512 bytes of
data to make its decision, and the code seems to agree with that comment.

But http.ServeContent (serveContent, really) reads twice that much. It should only read
the amount that DetectContentType is using.

[bradfitz]

Comment 1:

https://golang.org/cl/13289047

Owner changed to @bradfitz.

Status changed to Started.

[bradfitz]

Comment 2:

This issue was closed by revision 8272c14.

Status changed to Fixed.