Add repro project for SPR-12229

Add repro project for [SPR-12229](https://jira.spring.io/browse/SPR-12229)

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## Problem sample

run with

    $ mvn jetty:run

request with

    $ curl localhost:8080

Exception occurs like:

    2014-09-21 17:11:20,204 DEBUG [org.springframework.web.servlet.DispatcherServlet] - <Could not complete request>
    java.lang.UnsupportedOperationException
            at java.util.Collections$UnmodifiableMap.put(Collections.java:1517)
            at org.springframework.http.HttpHeaders.set(HttpHeaders.java:869)
            at org.springframework.http.converter.protobuf.ProtobufHttpMessageConverter.setProtoHeader(ProtobufHttpMessageConverter.java:196)
            at org.springframework.http.converter.protobuf.ProtobufHttpMessageConverter.writeInternal(ProtobufHttpMessageConverter.java:182)
            at org.springframework.http.converter.protobuf.ProtobufHttpMessageConverter.writeInternal(ProtobufHttpMessageConverter.java:60)
            at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:208)
            ... omitted


## Fixed sample

run with

    $ mvn jetty:run -Dfixed

request with

    $ curl localhost:8080
    
    John Doe�	jdoe@example.com"
    
    555-4321