Reverts "Set the default value of exposure_weight in createMergeMertens to 1.0"…

Reverts "Set the default value of exposure_weight in createMergeMertens to 1.0" because it breaks source code compatibility and tests
This reverts commit dbfd6a3d.

modules/photo/include/opencv2/photo.hpp

@@ -694,7 +694,7 @@ public:
 @param exposure_weight well-exposedness measure weight
  */
 CV_EXPORTS_W Ptr<MergeMertens>
-createMergeMertens(float contrast_weight = 1.0f, float saturation_weight = 1.0f, float exposure_weight = 1.0f);
+createMergeMertens(float contrast_weight = 1.0f, float saturation_weight = 1.0f, float exposure_weight = 0.0f);
 
 /** @brief The resulting HDR image is calculated as weighted average of the exposures considering exposure
 values and camera response.