Fix to write STRSXP when lengths of strings differ

* src/rmatio.c (write_strsxp): Added check to see if lenghts of
  strings are equal. If the lengths are non-equal, the strings are
  written to a cell. Need to fix if target is a mat_struct.

* src/rmatio.c (write_matvar): Prepared to add support compression, by
  adding the compression argument in write_matvar.