helper/schema: diff floats properly

/cc @phinze - This is pretty straightforward, almost magically so. The
reason this works is because in `diffString` we use mapstructure[1] with
"weak decode mode" to just be responisble for turning anything into a
string.

[1]: https://github.com/mitchellh/mapstructure

helper/schema/schema.go

@@ -466,6 +466,8 @@ func (m schemaMap) diff(
 		fallthrough
 	case TypeInt:
 		fallthrough
+	case TypeFloat:
+		fallthrough
 	case TypeString:
 		err = m.diffString(k, schema, diff, d, all)
 	case TypeList:

helper/schema/schema_test.go

@@ -1790,7 +1790,7 @@ func TestSchemaMap_Diff(t *testing.T) {
 
 			Diff: &terraform.InstanceDiff{
 				Attributes: map[string]*terraform.ResourceAttrDiff{
-					"port": &terraform.ResourceAttrDiff{
+					"some_threshold": &terraform.ResourceAttrDiff{
 						Old: "567.8",
 						New: "12.34",
 					},