add a 4x4 inverse case which is not handled by the current
ei_compute_inverse_in_size4_case (reported by mikola on IRC)

test/inverse.cpp

diff --git a/test/inverse.cpp b/test/inverse.cpp
index 54dab48..d619543 100644
--- a/test/inverse.cpp
+++ b/test/inverse.cpp
@@ -77,4 +77,10 @@
     CALL_SUBTEST( inverse(MatrixXf(8,8)) );
     CALL_SUBTEST( inverse(MatrixXcd(7,7)) );
   }
+  
+  // test some tricky cases for 4x4 matrices
+  VERIFY_IS_APPROX((Matrix4f() << 0,0,1,0, 1,0,0,0, 0,1,0,0, 0,0,0,1).finished().inverse(),
+                   (Matrix4f() << 0,1,0,0, 0,0,1,0, 1,0,0,0, 0,0,0,1).finished());
+  VERIFY_IS_APPROX((Matrix4f() << 1,0,0,0, 0,0,1,0, 0,0,0,1, 0,1,0,0).finished().inverse(),
+                   (Matrix4f() << 1,0,0,0, 0,0,0,1, 0,1,0,0, 0,0,1,0).finished());
 }