Add GenericDocument ctor overload to specify JSON type.

It unifies the interfaces with Value where kXXXType can be passed
into constructor.
It enables shortcut that helps to avoid extra SetXXX() call following
construction of a document.

include/rapidjson/document.h

@@ -1748,6 +1748,13 @@ public:
     typedef GenericValue<Encoding, Allocator> ValueType;    //!< Value type of the document.
     typedef Allocator AllocatorType;                        //!< Allocator type from template parameter.
 
+    GenericDocument(Type type, Allocator* allocator = 0, size_t stackCapacity = kDefaultStackCapacity, StackAllocator* stackAllocator = 0) :
+        GenericValue<Encoding, Allocator>(type),  allocator_(allocator), ownAllocator_(0), stack_(stackAllocator, stackCapacity), parseResult_()
+    {
+        if (!allocator_)
+            ownAllocator_ = allocator_ = RAPIDJSON_NEW(Allocator());
+    }
+
     //! Constructor
     /*! \param allocator        Optional allocator for allocating memory.
         \param stackCapacity    Optional initial capacity of stack in bytes.