Respect the encoding of the source [Bug #18827]

	Do not override the input string encoding at the time of preparation,
	the source encoding is not determined from the input yet.
	---
	 parse.y                  | 26 ++++++++++++++++----------
	 test/ruby/test_ast.rb    | 13 +++++++++++++
	 test/ruby/test_syntax.rb |  9 +++++++++
	 3 files changed, 38 insertions(+), 10 deletions(-)