Fix zero-length search string behavior in String#start_with?.

Fixes #2434.

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

@@ -5250,11 +5250,30 @@ public IRubyObject start_with_p(ThreadContext context, IRubyObject[]args) {
     }
 
     private boolean start_with_pCommon(IRubyObject arg) {
-        IRubyObject tmp = arg.checkStringType();
-        if (tmp.isNil()) return false;
-        RubyString otherString = (RubyString)tmp;
+        Ruby runtime = getRuntime();
+        RubyString otherString;
+
+        if (!runtime.is2_0()) {
+            // 1.8 and 1.9 ignores uncoercible argument
+            IRubyObject tmp = arg.checkStringType();
+            if (tmp.isNil()) return false;
+            otherString = (RubyString) tmp;
+        } else {
+            // 2.0+ requires coersion to succeed
+            otherString = arg.convertToString();
+        }
+
         checkEncoding(otherString);
+
+        int otherLength = otherString.value.getRealSize();
+
+        if (otherLength == 0) {
+            // other is '', so return true
+            return true;
+        }
+
         if (value.getRealSize() < otherString.value.getRealSize()) return false;
+
         return value.startsWith(otherString.value);
     }