Truncated JSON header value with @Headers

Observed on versions: 11.2, 11.4
Not reproducible on version: 10.1.0

When passing a JSON string as `@Param` to `@Headers` the string gets truncated.

Example: 
```
{"some": "value", "other": "value"}
```

Gets truncated to:
```
{"some": "value", "other"}
```

Code to reproduce: https://github.com/daczczcz1/feign-bug-reproduction