Literal \あ results in invalid multibyte char error #1390
Description
As reported in https://twitter.com/y_shindoh/status/421425572284465152.
test.rb
puts '\\あ'$ jruby -v --1.8 test.rb
jruby 1.7.9 (ruby-1.8.7p370) 2013-12-06 87b108a on Java HotSpot(TM) 64-Bit Server VM 1.7.0_45-b18 [darwin-x86_64]
\あ
$ jruby -v --1.9 -E UTF-8 test.rb
jruby 1.7.9 (1.9.3p392) 2013-12-06 87b108a on Java HotSpot(TM) 64-Bit Server VM 1.7.0_45-b18 [darwin-x86_64]
SyntaxError: test.rb:2: invalid multibyte char (US-ASCII)
$ jruby -v -E UTF-8 test.rb
jruby 1.7.10 (1.9.3p392) 2014-01-09 6e79f55 on Java HotSpot(TM) 64-Bit Server VM 1.7.0_45-b18 [darwin-x86_64]
SyntaxError: test.rb:2: invalid multibyte char (US-ASCII)
It works on (near) master in the default 2.1.0 mode.