message: add base64 0 buffer test
diff: #include "common/common/base64.h"
  TEST(Base64, EmptyBufferEncode) {
  + {
  Buffer::OwnedImpl buffer;
  EXPECT_EQ("", Base64::encode(buffer, 0));
  }
  + {
  + Buffer::OwnedImpl buffer;
  + buffer.add("\0\0");
  + EXPECT_EQ("", Base64::encode(buffer, 2));
  + }
  +}
  +
  TEST(Base64, SingleSliceBufferEncode) {
  Buffer::OwnedImpl buffer;
  buffer.add("foo", 3);
  