archive/tar: PAX does not take precedence over USTAR

[dsnet]

Using go1.6

According to PAX standard, PAX fields take precedence over USTAR fields. Thus, if the file size fields from USTAR and PAX differ, PAX should take precedence. This is especially critical for large file support with PAX.

Example:
https://play.golang.org/p/Oxk0gb-yoC

[gopherbot]

CL https://golang.org/cl/27454 mentions this issue.