py/formatfloat: Fix number of digits and exponent sign when rounding.

This patch fixes 2 things when printing a floating-point number that
requires rounding up of the mantissa:
- retain the correct precision; eg 0.99 becomes 1.0, not 1.00
- if the exponent goes from -1 to 0 then render it as +0, not -0

tests/float/string_format_modulo.py

@@ -44,3 +44,6 @@ print(('%.40g' % 1e-3)[:2])
 print(('%.40g' % 1e-4)[:2])
 
 print("%.0g" % 1) # 0 precision 'g'
+
+print('%.1e' % 9.99) # round up with positive exponent
+print('%.1e' % 0.999) # round up with negative exponent