Merge branch 'appendStringOp'

The StringBuilder.append method invoked was StringBuilder.append(Object) as
both String and Char are being returned. The implementation of
StringBuilder.append(Object) when applied to a Character is essentially:

* call Character.toString - which produces a one element char[] and then
  creates a new String with the char array, which in turn does a defensive copy
  of the buffer.
* the string created is then used to add to the StringBuilder

This PR changes JsonAST.appendEscapedString to use StringBuilder.append(Char)
which directly inserts the character into the buffer avoiding the operations
described above

core/json/src/main/scala/net/liftweb/json/JsonAST.scala

@@ -852,7 +852,7 @@ object JsonAST {
   private def appendEscapedString(buf: StringBuilder, s: String) {
     for (i <- 0 until s.length) {
       val c = s.charAt(i)
-      buf.append(c match {
+      val strReplacement = c match {
         case '"'  => "\\\""
         case '\\' => "\\\\"
         case '\b' => "\\b"
@@ -861,8 +861,16 @@ object JsonAST {
         case '\r' => "\\r"
         case '\t' => "\\t"
         case c if ((c >= '\u0000' && c < '\u0020')) => "\\u%04x".format(c: Int)
-        case c => c
-      })
+
+        case _ => ""
+      }
+
+      // Use Char version of append if we can, as it's cheaper.
+      if (strReplacement.isEmpty) {
+        buf.append(c)
+      } else {
+        buf.append(strReplacement)
+      }
     }
   }