FIX always keep case for relation labels

The RST-DT manual implies (pp. 42--44) that case in relation labels is meaningful.

Educe was using lowercase in two places:
* counting relation types,
* conversion from dependency to "phrase"-ish tree.

@kowey please remember to keep this on hold until we discuss the need for relaxed nuclearity with higher wisdom, as this proposal effectively cancels relaxed nuclearity out.