Too much white-space in parseRichText() v0.4

The method parseRichText() concatenates all text found in individual text runs with a white-space, i.e. by `implode(' ', $value)` on line 315 in v0.4. 

At least with our XLSX this turns out to be wrong since it adds white-space between differently formatted text elements in one cell and by that doubling existing white-space. Changing it to `implode('', $value)` solved the problem.