Add test to demonstrate tagging fallback.

Typo fix.

Fix issues in test.

Update test code.

bson/compatability_test.go

@@ -0,0 +1,54 @@
+package bson_test
+
+import (
+	"github.com/globalsign/mgo/bson"
+	. "gopkg.in/check.v1"
+)
+
+type mixedTagging struct {
+	First  string
+	Second string `bson:"second_field"`
+	Third  string `json:"third_field"`
+	Fourth string `bson:"fourth_field" json:"alternate"`
+}
+
+// TestTaggingFallback checks that tagging fallback can be used/works as expected.
+func (s *S) TestTaggingFallback(c *C) {
+	initial := &mixedTagging{
+		First:  "One",
+		Second: "Two",
+		Third:  "Three",
+		Fourth: "Four",
+	}
+
+	// Take only testing.T, leave only footprints.
+	initialState := bson.JSONTagFallbackState()
+	defer bson.SetJSONTagFallback(initialState)
+
+	// Marshal with the new mode applied.
+	bson.SetJSONTagFallback(true)
+	bson, errBSON := bson.Marshal(initial)
+	c.Assert(errBSON, IsNil)
+
+	// Unmarshal into a generic map so that we can pick up the actual field names
+	// selected.
+	target := make(map[string]string)
+	errUnmarshal := bson.Unmarshal(bson, target)
+	c.Assert(errUnmarshal, IsNil)
+
+	// No tag, so standard naming
+	_, firstExists := target["first"]
+	c.Assert(firstExists, Exists, true)
+
+	// Just a BSON tag
+	_, secondExists := target["second_field"]
+	c.Assert(secondExists, Exists, true)
+
+	// Just a JSON tag
+	_, thirdExists := target["third_field"]
+	c.Assert(thirdExists, Exists, true)
+
+	// Should marshal 4th as fourth_field (since we have both tags)
+	_, fourthExists := target["fourth_field"]
+	c.Assert(fourthExists, Exists, true)
+}