Change struct decoder to be case-insensitive.

GODRIVER-1011

Change-Id: I31674da7af7405f659dbbe7d0aaeeadca7fdb946

Author: Divjot Arora

bson/bsoncodec/struct_codec.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"reflect"
+	"strings"
 	"sync"
 
 	"go.mongodb.org/mongo-driver/bson/bsonrw"
@@ -160,6 +161,13 @@ func (sc *StructCodec) DecodeValue(r DecodeContext, vr bsonrw.ValueReader, val r
 		}
 
 		fd, exists := sd.fm[name]
+		if !exists {
+			// if the original name isn't found in the struct description, try again with the name in lowercase
+			// this could match if a BSON tag isn't specified because by default, describeStruct lowercases all field
+			// names
+			fd, exists = sd.fm[strings.ToLower(name)]
+		}
+
 		if !exists {
 			if sd.inlineMap < 0 {
 				// The encoding/json package requires a flag to return on error for non-existent fields.

bson/unmarshaling_cases_test.go

@@ -64,4 +64,17 @@ var unmarshalingTestCases = []unmarshalingTestCase{
 		},
 		docToBytes(D{{"foo", A{true}}}),
 	},
+	{
+		"struct with mixed case fields",
+		nil,
+		reflect.TypeOf(struct {
+			FooBar int32
+		}{}),
+		&struct {
+			FooBar int32
+		}{
+			FooBar: 10,
+		},
+		docToBytes(D{{"fooBar", int32(10)}}),
+	},
 }