Add missing default ctor in Rotation2D

Eigen/src/Geometry/Rotation2D.h

diff --git a/Eigen/src/Geometry/Rotation2D.h b/Eigen/src/Geometry/Rotation2D.h
index c82762a..776c361 100644
--- a/Eigen/src/Geometry/Rotation2D.h
+++ b/Eigen/src/Geometry/Rotation2D.h
@@ -60,6 +60,9 @@
 
   /** Construct a 2D counter clock wise rotation from the angle \a a in radian. */
   explicit inline Rotation2D(const Scalar& a) : m_angle(a) {}
+  
+  /** Default constructor wihtout initialization. The represented rotation is undefined. */
+  Rotation2D() {}
 
   /** \returns the rotation angle */
   inline Scalar angle() const { return m_angle; }
@@ -81,10 +84,10 @@
   /** Applies the rotation to a 2D vector */
   Vector2 operator* (const Vector2& vec) const
   { return toRotationMatrix() * vec; }
-
+  
   template<typename Derived>
   Rotation2D& fromRotationMatrix(const MatrixBase<Derived>& m);
-  Matrix2 toRotationMatrix(void) const;
+  Matrix2 toRotationMatrix() const;
 
   /** \returns the spherical interpolation between \c *this and \a other using
     * parameter \a t. It is in fact equivalent to a linear interpolation.

test/geo_transformations.cpp

diff --git a/test/geo_transformations.cpp b/test/geo_transformations.cpp
index 7d90803..1768d7b 100644
--- a/test/geo_transformations.cpp
+++ b/test/geo_transformations.cpp
@@ -397,6 +397,15 @@
   t20 = Translation2(v20) * (Rotation2D<Scalar>(s0) * Eigen::Scaling(s0));
   t21 = Translation2(v20) * Rotation2D<Scalar>(s0) * Eigen::Scaling(s0);
   VERIFY_IS_APPROX(t20,t21);
+  
+  // check basic features
+  {
+    Rotation2D<Scalar> r1;           // default ctor
+    r1 = Rotation2D<Scalar>(s0);     // copy assignment
+    VERIFY_IS_APPROX(r1.angle(),s0);
+    Rotation2D<Scalar> r2(r1);       // copy ctor
+    VERIFY_IS_APPROX(r2.angle(),s0);
+  }
 }
 
 template<typename Scalar> void transform_alignment()