webrick: compile RE correctly for beginning and end match

	Using ^ and $ in regexps means we can accidentally get fooled
	by "%0a" in HTTP request paths being decoded to newline
	characters.  Use \A and \z to match beginning and end-of-string
	respectively, instead.

	Thanks to mame and hsbt for reporting.

	* lib/webrick/httpserver.rb (MountTable#compile):
	  use \A and \z instead of ^ and $
	* lib/webrick/httpserver.rb (MountTable#normalize): use \z instead of $
	* test/webrick/test_httpserver.rb (test_cntrl_in_path): new test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@61238 b2dd03c8-39d4-4d8f-98ff-823fe69b080e