Make "" mode being the same as debug mode (gin-gonic#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 "".

(cherry picked from commit 5d3f30c)

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())