date_strptime.c: get rid of backtrack explosion

* ext/date/date_strptime.c (date__strptime_internal): unset
  case-insensitive flag for [:alpha:], which already implies both
  cases, to get rid of backtrack explosion.  [ruby-core:58984]
  [Bug #9221]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44126 b2dd03c8-39d4-4d8f-98ff-823fe69b080e