Fixes jruby#7649. Infinite loop in StringScanner regex

Crux of problem was strscan#concat was calling JRuby String append
function in Java and that was a pre-m17n version of append which
was not encoding aware OR coderange aware.  As a result, the string
thought it was 7bit ASCII (narrator: it was not) and this made
joni go off the rails.

The fix I used was to move append19 impl into append and deprecate
append19.  This version does all the right things.  In looking
at callers of append there were not many consumers.  ARGF#read
was also broken in the same way since we would be appending what-
ever file in its encoding onto the existing string.

core/src/main/java/org/jruby/RubyArray.java

@@ -1968,7 +1968,7 @@ protected RubyString joinStrings(RubyString sep, int max, RubyString result) {
         try {
             for (int i = 0; i < max; i++) {
                 if (i > 0 && sep != null) result.cat19(sep);
-                result.append19(eltInternal(i));
+                result.append(eltInternal(i));
             }
         } catch (ArrayIndexOutOfBoundsException e) {
             throw concurrentModification(getRuntime(), e);

core/src/main/java/org/jruby/RubyString.java

@@ -2640,32 +2640,7 @@ public boolean isEmpty() {
      *
      */
     public RubyString append(IRubyObject other) {
-        modifyCheck();
-
-        if (other instanceof RubyFixnum) {
-            cat(ConvertBytes.longToByteList(((RubyFixnum) other).value));
-            return this;
-        }
-        if (other instanceof RubyFloat) {
-            return cat((RubyString) ((RubyFloat) other).to_s());
-        }
-        if (other instanceof RubySymbol) {
-            cat(((RubySymbol) other).getBytes());
-            return this;
-        }
-        RubyString otherStr = other.convertToString();
-        infectBy(otherStr);
-        return cat(otherStr.value);
-    }
-
-    public RubyString append(RubyString other) {
-        modifyCheck();
-        infectBy(other);
-        return cat(other.value);
-    }
-
-    public RubyString append19(IRubyObject other) {
-        // fast path for fixnum straight into ascii-compatible bytelist
+        // fast path for fixnum straight into ascii-compatible bytelist (modify check performed in here)
         if (other instanceof RubyFixnum && value.getEncoding().isAsciiCompatible()) {
             ConvertBytes.longIntoString(this, ((RubyFixnum) other).value);
             return this;
@@ -2683,6 +2658,17 @@ public RubyString append19(IRubyObject other) {
         return cat19(other.convertToString());
     }
 
+    public RubyString append(RubyString other) {
+        modifyCheck();
+        infectBy(other);
+        return cat19(other);
+    }
+
+    @Deprecated
+    public RubyString append19(IRubyObject other) {
+        return append(other);
+    }
+
     public RubyString appendAsDynamicString(IRubyObject other) {
         // fast path for fixnum straight into ascii-compatible bytelist
         if (other instanceof RubyFixnum && value.getEncoding().isAsciiCompatible()) {