Fix typo from expornent to exponent

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
fix TypeError message
fix codegen bug in NODE_CASE
Merge pull request #1610 from mruby-debian/for-upstream

Fix typo from expornent to exponent
Merge pull request #1611 from h2so5/inspect-type

fix TypeError message
Merge pull request #1612 from h2so5/node-case

fix codegen bug in NODE_CASE