Allow equal lower and upper bound for diagonal scaling.

This way, setting the lower and upper bound both to 1.0, one can disable
the automatic trust region scaling.

Change-Id: Ifa317a6911b813a89c1cf7fdfde25af603705319

internal/ceres/dogleg_strategy.cc

diff --git a/internal/ceres/dogleg_strategy.cc b/internal/ceres/dogleg_strategy.cc
index 131de39..668fa54 100644
--- a/internal/ceres/dogleg_strategy.cc
+++ b/internal/ceres/dogleg_strategy.cc
@@ -65,7 +65,7 @@
       dogleg_type_(options.dogleg_type) {
   CHECK_NOTNULL(linear_solver_);
   CHECK_GT(min_diagonal_, 0.0);
-  CHECK_LT(min_diagonal_, max_diagonal_);
+  CHECK_LE(min_diagonal_, max_diagonal_);
   CHECK_GT(max_radius_, 0.0);
 }
 

internal/ceres/levenberg_marquardt_strategy.cc

diff --git a/internal/ceres/levenberg_marquardt_strategy.cc b/internal/ceres/levenberg_marquardt_strategy.cc
index af76c42..9e6a59e 100644
--- a/internal/ceres/levenberg_marquardt_strategy.cc
+++ b/internal/ceres/levenberg_marquardt_strategy.cc
@@ -54,7 +54,7 @@
       reuse_diagonal_(false) {
   CHECK_NOTNULL(linear_solver_);
   CHECK_GT(min_diagonal_, 0.0);
-  CHECK_LT(min_diagonal_, max_diagonal_);
+  CHECK_LE(min_diagonal_, max_diagonal_);
   CHECK_GT(max_radius_, 0.0);
 }