Fix block argument destructuring with optional parameters

Author: evaniainbrooks

core/src/main/java/org/jruby/runtime/IRBlockBody.java

@@ -161,7 +161,7 @@ public IRubyObject doYield(ThreadContext context, Block block, IRubyObject value
         IRubyObject[] args;
         if (value == null) { // no args case from BlockBody.yieldSpecific
             args = IRubyObject.NULL_ARRAY;
-        } else if (!signature.isSpreadable()) {
+        } else if (!signature.isSpreadable() || (signature.opt() + signature.required() == 1 && !signature.hasRest())) {
             args = new IRubyObject[] { value };
         } else {
             args = toAry(context, value);

spec/ruby/language/yield_spec.rb

@@ -48,6 +48,12 @@
       it "passes a single, multi-value Array" do
         @y.s([1, 2, 3]) { |*a| a }.should == [[1, 2, 3]]
       end
+
+      describe "with optional argument" do
+        it "does not destructure a single array argument" do
+          @y.s([1, 2, 3]) { |a = 99| a }.should == [1, 2, 3]
+        end
+      end
     end
 
     describe "yielding to a lambda" do