</target>
            </configuration>
          </execution>
        </executions>
      </plugin>
      <!-- TODO(cpovirk): We really should try tbroyer's plugin instead of this legacy one. -->
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>gwt-maven-plugin</artifactId>
        <version>${gwt.plugin.version}</version>
        <executions>
          <execution>

HTTP headers work like a `Map<String, String>`: each field has one value or none. But some headers permit multiple values, like Guava's [Multimap](https://guava.dev/releases/23.0/api/docs/com/google/common/collect/Multimap.html). For example, it's legal and common for an HTTP response to supply multiple `Vary` headers. OkHttp's APIs attempt to make both cases comfortable.

When writing request headers, use `header(name, value)` to set the only occurrence of `name` to `value`. If there are...