add mrb_string_type().
Check type of arguments for #start_with and #end_with.
Merge branch 'mruby' into pr-typeerror-string-start-end-with

Conflicts:
	mrbgems/mruby-string-ext/src/string.c
	mrbgems/mruby-string-ext/test/string.rb
Merge pull request #1486 from iij/pr-typeerror-string-start-end-with

Crash if you call String#start_with/end_with with non-String arguments.