Made it so you can say sgd(weight_decay) to construct a sgd object.

dlib/dnn/solvers.h

@@ -14,9 +14,9 @@ namespace dlib
     {
     public:
 
-        sgd(
+        explicit sgd(
             float weight_decay_,
-            float momentum_ 
+            float momentum_ = 0.9
         ) 
         { 
             weight_decay = weight_decay_;

dlib/dnn/solvers_abstract.h

@@ -103,9 +103,9 @@ namespace dlib
                 - #get_momentum()      == 0.9 
         !*/
 
-        sgd(
+        explicit sgd(
             float weight_decay,
-            float momentum 
+            float momentum = 0.9
         ); 
         /*!
             requires