enumerator.c: fix arith_seq_first for Infinity

	* enumerator.c (arith_seq_first): fix for Float::INFINITY.

	* test/ruby/test_arithmetic_sequence.rb: add tests.

	* numeric.c (ruby_float_step_size): export for internal use.

	* internal.h: add prototype declaration of ruby_float_step_size.

	[ruby-core:90937][Bug #15518]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66949 b2dd03c8-39d4-4d8f-98ff-823fe69b080e