Fixed check for empty multipart/related document

Ignore-this: ee48fc69ad808a44da2cbcf3f6ea1d69

darcs-hash:20120903141301-b22ec-0a612131be779a84710d9f5b74a0c73ab021ace7
Renamed variable

Ignore-this: 6e450a59d3b9267ab318a36a1170369

darcs-hash:20120903141558-b22ec-c408bc958ed649d9141c2fc09040d70d9111a130