HTMLEncode strings in wddx_serialize_value()

Summary: Strings returned through wddx_serialize_value should be HTMLEncode()'d during
serialization.

Fixes #4283

{sync, type="child", parent="internal", parentrevid="1691695", parentrevfbid="1537976659780590", parentdiffid="5726084"}

Reviewed By: @JoelMarcey

Differential Revision: D1691695

Signature: t1:1691695:1416530595:722bfcdaf7c0dbee379bea886cd4c43d997ca7dd