Document JSON marshaling issue with escaped characters in `MarshalJSON` and update tests to prevent improper marshaling.

pkg/encoders/event/event.go

@@ -87,6 +87,11 @@ func (ev *E) Free() {
 // MarshalJSON marshals an event.E into a JSON byte string.
 //
 // Call bufpool.PutBytes(b) to return the buffer to the bufpool after use.
+//
+// WARNING: if json.Marshal is called in the hopes of invoking this function on
+// an event, if it has <, > or * in the content or tags they are escaped into
+// unicode escapes and break the event ID. Call this function directly in order
+// to bypass this issue.
 func (ev *E) MarshalJSON() (b []byte, err error) {
 	b = bufpool.Get()
 	b = b[:0]

pkg/encoders/event/event_test.go

@@ -78,10 +78,11 @@ func TestExamplesCache(t *testing.T) {
 		c = c[:0]
 		c = append(c, b...)
 		ev := New()
-		if err = ev.UnmarshalJSON(b); chk.E(err) {
+		if err = json.Unmarshal(b, ev); chk.E(err) {
 			t.Fatal(err)
 		}
 		var b2 []byte
+		// can't use json.Marshal as it improperly escapes <, > and &.
 		if b2, err = ev.MarshalJSON(); err != nil {
 			t.Fatal(err)
 		}