BUG: gh-2935, fix printing of scalar float -0.0.

The issue was that the string value of scalar floating types equal to -0.0
lacked a trailing ".0".

    >>> print np.float64(-0.0)
    -0

The problem resulted from using val < 0 to check for a leading negative
sign. That expression evaluates false when val = -0.0, and the need to
append ".0" was not detected.
TST: Add test for printing of scalar values.
Merge pull request #2936 from charris/fix-print-negative-float-zero

Fix print negative float zero