Issue title: Ruby19Parser doesn't handle string literals with Unicode escape sequences
 Issue body: test case:

``` ruby
require 'ruby_parser'

source = '"Se\xc3\xb1or"'
p eval(source)
p Ruby19Parser.new.parse(source)

source = '"Se\u00F1or"'
p eval(source)
p Ruby19Parser.new.parse(source)

source = '"Se\u{F1}or"'
p eval(source)
p Ruby19Parser.new.parse(source)
```

Expected Output:

```
"Se\xC3\xB1or"
s(:str, "Señor")
"Señor"
s(:str, "Señor")
"Señor"
s(:str, "Señor")
```

Actual Output:

```
"Se\xC3\xB1or"
s(:str, "Señor")
"Señor"
s(:str, "Seu00F1or")
"Señor"
s(:str, "Seu{F1}or")
```

 Comments: 
 Comment 0: duplicate of #98, #133 

 Comment 1: indeed a dupe

{'source': 'GithubIssues', 'repo_name_with_owner': 'seattlerb/ruby_parser', 'issue_id': 22188886, 'repo_stars': 449.0}