The error checks for both usage should be separated; ref #3774

Merge branch 'super_method_missing_fix' of https://github.com/christopheraue/mruby into christopheraue-super_method_missing_fix

And the function does not conform the naming convention anyway.

Updated AUTHORS

`flo_hash` implemented `15.2.8.3.18` but `Kernel#hash` (15.3.1.3.15)
now works for numbers.

Replaced tabs with spaces

When the method takes variable number of argument (`argc = -1`),
the block argument offset should be `-1` not `0`.

It also fixes unexpected resurrection of #3745 by #3752

Improved speed of creating new hash tables