Fixes #67: No type argument for 'to' = no crash.

Expected type is not propagated in explicit conversion to case classes
because we use whitebox macros. We could materialize Convert and
capture expected type (if present) but a) it seriously complicates the
implementation for little gain b) we potentially lose nice error
messages.
Merge pull request #69 from hubertp/issue-67

Fixes #67: No type argument for 'to' = no crash.