responses that can't be decoded into a map don't work

[dzfranklin]

Describe the bug

evans fails if the response can't be decoded into a map. for example for a google.protobuf.StringValue I see

command call: json: cannot unmarshal string into Go value of type map[string]interface {}

To reproduce

call an rpc method that returns a google.protobuf.StringValue

Expected behavior

I see the string value printed

Environment

• OS: Linux
• Terminal: alacritty
• Evans version: 0.10.11
• protoc version: 3.12.4
• protoc plugin version (if you are using):

Additional context

I believe the root cause is that the formatter attempts to decode the message into a map[string]interface{} and then json print that, which obviously fails on a string.

https://github.com/ktr0731/evans/blob/fb471a3a164a45f90d89cb1e56f4b9bdc54491c9/format/json/json.go#L110