Add a spec for BigDecimal#hash.

Author: nirvdrum

spec/ruby/library/bigdecimal/hash_spec.rb

@@ -0,0 +1,30 @@
+require_relative '../../spec_helper'
+require 'bigdecimal'
+
+describe "BidDecimal#hash" do
+  describe "two BigDecimal objects with the same value" do
+    it "should have the same hash for ordinary values" do
+      BigDecimal('1.2920').hash.should == BigDecimal('1.2920').hash
+    end
+
+    it "should have the same hash for infinite values" do
+      BigDecimal("+Infinity").hash.should == BigDecimal("+Infinity").hash
+      BigDecimal("-Infinity").hash.should == BigDecimal("-Infinity").hash
+    end
+
+    it "should have the same hash for NaNs" do
+      BigDecimal("NaN").hash.should == BigDecimal("NaN").hash
+    end
+
+    it "should have the same hash for zero values" do
+      BigDecimal("+0").hash.should == BigDecimal("+0").hash
+      BigDecimal("-0").hash.should == BigDecimal("-0").hash
+    end
+  end
+
+  describe "two BigDecimal objects with numerically equal values" do
+    it "should have the same hash value" do
+      BigDecimal("1.2920").hash.should == BigDecimal("1.2920000").hash
+    end
+  end
+end

src/main/java/org/truffleruby/stdlib/bigdecimal/BigDecimalNodes.java

@@ -1566,7 +1566,7 @@ public Object hashNormal(DynamicObject value) {
         public Object hashSpecial(DynamicObject value) {
             final BigDecimalType type = Layouts.BIG_DECIMAL.getType(value);
 
-            return getContext().getHashing(this).hash(CLASS_SALT, System.identityHashCode(type));
+            return getContext().getHashing(this).hash(CLASS_SALT, type.hashCode());
         }
 
     }