insert spaces for TextElement distances slightly less than width_of_space

I found part of the problem with spaces not being inserted that I'd had in tabula-extractor. Some distances between text_elements in `should_add_space?` are reporting themselves as a little less than width_of_space when (from a human perspective) there ought to be a space.

this could be  

1. a pdfbox bug,  
2. a conceptual problem we have where the width_of_space changes in the document
3. kerning 
4.  something else entirely

e.g. I'm getting distances in the frx test document (in the table-detection branch) of 1.6399999999999864 with width_of_space 1.668
so a space isn't inserted because 1.64 < 1.668

this PR fixes the problem by inserting a space when the distance is between 95% of width_of_space and width_of_space plus the threshold. 95% wasn't reached scientifically, but gives perfect results empirically.