encoding/xml: only initialize nil struct fields when decoding
fieldInfo.value used to initialize nil anonymous struct fields if they
were encountered. This behavior is wanted when decoding, but not when
encoding. When encoding, the value should never be modified, and these
nil fields should be skipped entirely.
To fix the bug, add a bool argument to the function which tells the
code whether we are encoding or decoding.
Finally, add a couple of tests to cover the edge cases pointed out in
the original issue.