Don't zero out default log package flags.

[rdwilliamson]

I imported a package (github.com/mkevac/debugcharts) that has dependencies on gin. Suddenly all my log messages were missing the time stamps and I tracked it down to the init function in debug.go

[aronatkins]

Also encountered this issue with to gin calling log.SetFlags. Working around by explicitly restoring the standard logging flags in my application main.

log.SetFlags(log.LstdFlags)

[thinkerou]

please see #1560 and close.