GenericValue: fix comparison of (Ui|I)nt64 numbers

Some 64-bit integers cannot be represented losslessly as a double.
Due to a typo in the operator==, the comparison has been performed
after a double conversion in too many cases.

include/rapidjson/document.h

@@ -662,7 +662,7 @@ public:
             return StringEqual(rhs);
 
         case kNumberType:
-            if (IsDouble() || rhs.GetDouble())
+            if (IsDouble() || rhs.IsDouble())
                 return GetDouble() == rhs.GetDouble(); // May convert one operand from integer to double.
             else
                 return data_.n.u64 == rhs.data_.n.u64;