Don't skip over multiple spaces when '' (blank) is an NA string

[dvg-p4]

Closes #3658

TODO:

• Fix the failing existing tests (not sure if the tests are expecting the wrong behavior or if I introduced a bug)
• Write a test for the intended behavior
• Explain the existing space-condensing behavior and the new exception to it in fread's documentation

[dvg-p4]

Hmm, so it looks like (part) of the issue is that strip.white = TRUE (by default) for that test; and the other part of the issue is that the whitespace-stripping code isn't accounting for this change I made to not-skip-spaces. I don't think it's super clear what should be done with fread(sep = ' ', na.strings = '', strip.white = TRUE); it's a bit contradictory.

[dvg-p4]

I think given that strip.white = TRUE is the default, the correct behavior is:

If sep is specified or detected to be ' ' [space], na.strings includes '' [blank], and strip.white is TRUE; warn the user, set stripWhite to false, and continue.

[dvg-p4]

Ugh, this is even more of a mess than I realized. In some places, we're defining whitespace as ' ', in others as whiteChar [with 0 meaning "both ' ' and '\t'"], in others by isspace()...

and deciding whether to strip it sometimes by whether stripWhite is true, and other times by whether sep is ' '...