This matches the old JSON package behavior.
All lowercase names are not as standard as I believed,
and it seems less surprising to need to write
type T struct { Field string "field" }
to get lower case (behavior after this CL) than it does to need
to write
type T struct { Field string "Field" }
to preserve the case (behavior before this CL).
Also test and fix unmarshal into non-nil interface
value or pointer.
Fixes#744.
R=r
CC=golang-dev
https://golang.org/cl/1013041
