Encoding.Decode() failed to detect trailing garbages if input contains "==" followed by garbage smaller than 3 bytes (for example, it failed to detect "x" in "AA==x"). This patch fixes the bug and adds a few tests.

LGTM=nigeltao
R=golang-codereviews, bradfitz, nigeltao
CC=golang-codereviews
https://golang.org/cl/75340044