fixing test case + adding migration docs

docs/migrations/v3-to-v4.md

@@ -0,0 +1,26 @@
+# Migrating from ibc-go v2 to v3
+
+This document is intended to highlight significant changes which may require more information than presented in the CHANGELOG.
+Any changes that must be done by a user of ibc-go should be documented here.
+
+There are four sections based on the four potential user groups of this document:
+- Chains
+- IBC Apps
+- Relayers
+- IBC Light Clients
+
+**Note:** ibc-go supports golang semantic versioning and therefore all imports must be updated to bump the version number on major releases.
+```go
+github.com/cosmos/ibc-go/v2 -> github.com/cosmos/ibc-go/v3
+```
+
+No genesis or in-place migrations required when upgrading from v1 or v2 of ibc-go.
+
+## Chains
+
+### IS04 - Channel 
+
+The `WriteAcknowledgement` API now takes the `exported.Acknowledgement` type instead of passing in the acknowledgement byte array directly. 
+This is an API breaking change and as such IBC application developers will have to update any calls to `WriteAcknowledgement`. 
+
+

modules/core/04-channel/keeper/packet_test.go

@@ -551,8 +551,7 @@ func (suite *KeeperTestSuite) TestWriteAcknowledgement() {
 			func() {
 				suite.coordinator.Setup(path)
 				packet = types.NewPacket(ibctesting.MockPacketData, 1, path.EndpointA.ChannelConfig.PortID, path.EndpointA.ChannelID, path.EndpointB.ChannelConfig.PortID, path.EndpointB.ChannelID, timeoutHeight, disabledTimeoutTimestamp)
-				ack = types.NewResultAcknowledgement([]byte{})
-				suite.chainB.App.GetIBCKeeper().ChannelKeeper.SetPacketAcknowledgement(suite.chainB.GetContext(), packet.GetDestPort(), packet.GetDestChannel(), packet.GetSequence(), ack.Acknowledgement())
+				ack = types.NewResultAcknowledgement(nil)
 				channelCap = suite.chainB.GetChannelCapability(path.EndpointB.ChannelConfig.PortID, path.EndpointB.ChannelID)
 			},
 			false,