protoc: does not verify that json_name does not conflict

Using `protoc v3.6.1`.

Compile this file:
```proto 
syntax = "proto3";
message M {
	string f1 = 1 [json_name = "fooBar"];
	string f2 = 2 [json_name = "fooBar"];
}
```

What I see: compile succeeds.
What I expect: compilation failure since `fooBar` is specified twice.