oox: whitespace fixes in WpsContext

Change-Id: Idcd4409a02a5b0dc5f7b67d2a482bf4fe30f579c
fdo#65090 RTF filter: import RTF_CLMGF and RTF_CLMRG

These describe an explicit horizontal merge, that is not something Word
itself creates, but it turns out the Calc RTF export does.

Change-Id: I1b6ec10bb8e8bd40e24791ccc96f2f066dd0d5d5