ENH: Corrects to_html print spacing GH4987
Closes #4987 

This is my first non-doc PR, so please bare with me.  Is it worth my while to go through the tests and standarize all of the test HTML to all block-strings, or is the mix of individual lines/blocks ok?

does this actually change the way the output looks? 
 As far as I know it only strips off the leading/trailing spaces in the HTML data rows. Those spaces shouldn't be rendered by a browser?
 @jorisvandenbossche @cpcloud @TomAugspurger look ok?
 @clham can you add a release note? (put it under API changes section in whatsnew, just a single line and reference the underlying issue)
  @jreback, Should be good to go. I added a plain api section. Current version only
lists breakages.
 @TomAugspurger @jorisvandenbossche any problem with this?
 Seems OK to me
 But I think this should not be listed in API changes, but can be considered as a bug fix. 
And it needs a rebase.
 merged via https://github.com/pydata/pandas/commit/ed45e6eeab43c1abf7b541fd659dada12172b52c

thanks!
 Thanks a lot for this, I had some problems letting pandoc convert such html and was doing a `re.sub(' +',' ', html)`but after this it isn't nessesary anymore :-)
