more mrb_flo_to_str fixes

- remove float check (take mrb_float instead of mrb_value)
- support -0.0
- fix range error if log10 is called when n == 0.0
- MSVC older than 2013: add signbit macro (for double)
mrb_flo_to_str: internal linkage
clarify fallthrough in the switch statement
resolve conflict
Merge branch 'master' of github.com:mruby/mruby