Fix #174

[JRUBY-6668] StringScanner#scan_until spins forever on UTF-8 data

We were not preparing the regex properly. Added that, and the
given example completes normally.

src/org/jruby/RubyRegexp.java

@@ -396,7 +396,7 @@ private Encoding checkEncoding(RubyString str, boolean warn) {
         return enc;
     }
 
-    final Regex preparePattern(RubyString str) {
+    public final Regex preparePattern(RubyString str) {
         check();
         Encoding enc = checkEncoding(str, true);
         if (enc == pattern.getEncoding()) return pattern;

src/org/jruby/ext/strscan/RubyStringScanner.java

@@ -227,10 +227,11 @@ private IRubyObject extractBegLen(Ruby runtime, int beg, int len) {
     }
 
     private IRubyObject scan(IRubyObject regex, boolean succptr, boolean getstr, boolean headonly) {
-        if (!(regex instanceof RubyRegexp)) throw getRuntime().newTypeError("wrong argument type " + regex.getMetaClass() + " (expected Regexp)");
+        Ruby runtime = getRuntime();
+        if (!(regex instanceof RubyRegexp)) throw runtime.newTypeError("wrong argument type " + regex.getMetaClass() + " (expected Regexp)");
         check();
 
-        Regex pattern = ((RubyRegexp)regex).getPattern();
+        Regex pattern = ((RubyRegexp)regex).preparePattern(str);
 
         clearMatched();
         int rest = str.getByteList().getRealSize() - pos;