merge revision(s) 66745,67397,67398,67399,67400,67409,67410,67411,67412,67425,67426,67473,67474: [Backport #15742]

	Bump version to date-2.0.0.

	  I forgot to change it when Ruby 2.6.0 was released.

	date_parse.c: extract Japanese era initials

	* expand tabs.

	date_parse.c: removed 'r' which is not in JIS X 0301 yet

	Added tests for end of Heisei

	date: use del_hash to extract an element destructively

	* expand tabs.

	date_parse.c: renamed JAPANESE prefix as JISX0301

	date_parse.c: name JISX0301_DEFAULT_ERA

	date: make zone a substring to copy encoding and taintedness

	* expand tabs.

	date_core.c: [DOC] Heisei will be assumed if no-era [ci skip]

	date: support for Reiwa, new Japanese era

	[Feature #15742]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67527 b2dd03c8-39d4-4d8f-98ff-823fe69b080e