Exception when adding new value to an existing header through StandaloneAhcWSRequest::addHeader

Play Version

2.6.6

API

Java

Operating System

Ubuntu 16.04 LTS
Linux 4.4.0-101-generic #124-Ubuntu SMP Fri Nov 10 18:29:59 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

JDK

openjdk version "1.8.0_151"
OpenJDK Runtime Environment (build 1.8.0_151-8u151-b12-0ubuntu0.16.04.2-b12)
OpenJDK 64-Bit Server VM (build 25.151-b12, mixed mode)

Library Dependencies

javaWs

Expected Behavior

1. I'm using StandaloneAhcWSRequest to add a new value for an existing header to a given request. I call the method StandaloneAhcWSRequest::addHeader, line 100 in StandaloneAhcWSRequest.class.
2. This method calls the method StandaloneAhcWSRequest::addValueTo, line 505.
3. Then it manages the Map<String, List<String>> headers. If the new header already existed, it takes the list and adds the new value to it. Therefore, if for example we have the header myHeader already existing in headers map with a value1 and we want to add a value2 to that header, in the end it should have: <"myHeader", ["value1", "value2"]>

Actual Behavior

1. I get UnsupportedOperationException.
2. The reason is that the implementation used for the list which is going to get the new element added is the scala one. In scala the list is immutable.

Stack trace

...
play.core.j.HttpExecutionContext$$anon$2.run(HttpExecutionContext.scala:56)
... 6 common frames omitted
Caused by: java.lang.UnsupportedOperationException: null
at java.util.AbstractList.add(AbstractList.java:148)
at java.util.AbstractList.add(AbstractList.java:108)
at play.libs.ws.ahc.StandaloneAhcWSRequest.addValueTo(StandaloneAhcWSRequest.java:508)
at play.libs.ws.ahc.StandaloneAhcWSRequest.addHeader(StandaloneAhcWSRequest.java:101)
at play.libs.ws.ahc.StandaloneAhcWSRequest.addHeader(StandaloneAhcWSRequest.java:41) 
...

Start with play.libs.ws.ahc.StandaloneAhcWSRequest.addHeader and addValueTo, identified at lines 101 and 508 in the issue, and inspect how an existing header's values list is updated. Confirm that adding a second value no longer throws UnsupportedOperationException and preserves both header values.