merge revision(s) r45207,r45208,r45209,r45210: [Backport #9575]

	* numeric.c: Create var for rb_intern("<=>")
	* numeric.c: Fix Numeric#step with 0 unit [Bug #9575]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46401 b2dd03c8-39d4-4d8f-98ff-823fe69b080e