Make "" mode being the same as debug mode (#1250)

Not setting mode explicitly sets gin into debug mode, but it does not
make it possible to retrieve gin mode as Debug since it's set to "".

mode.go

@@ -53,6 +53,9 @@ func SetMode(value string) {
 	default:
 		panic("gin mode unknown: " + value)
 	}
+	if value == "" {
+		value = DebugMode
+	}
 	modeName = value
 }
 

mode_test.go

@@ -21,6 +21,10 @@ func TestSetMode(t *testing.T) {
 	assert.Equal(t, TestMode, Mode())
 	os.Unsetenv(ENV_GIN_MODE)
 
+	SetMode("")
+	assert.Equal(t, debugCode, ginMode)
+	assert.Equal(t, DebugMode, Mode())
+
 	SetMode(DebugMode)
 	assert.Equal(t, debugCode, ginMode)
 	assert.Equal(t, DebugMode, Mode())