SpringCodegen not importing java.util.Map

Description

When using the codegen maven plugin and the Spring language, java.util.Map is not being imported when the Map's value is a List.

Swagger-codegen version

Maven plugin version 2.3.1

Swagger declaration file content or url

paths:
  /customer:
    get:
      tags:
        - Customers
      summary: Retrieve all customers
      produces:
        - application/json
      responses:
        '200':
          description: OK
          schema:
            type: object
            additionalProperties:
              type: array
              items:
                type: string

the output of this is correct

default ResponseEntity<Map<String, List<String>>>

but the class does not have java.util.Map imported, and therefore fails to build

target/generated-sources/swagger/src/gen/java/main/io/swagger/api/CustomerApi.java:[54,28] cannot find symbol
  symbol:   class Map
  location: interface io.swagger.api.CustomerApi

Maven configuration

<plugin>
                <groupId>io.swagger</groupId>
                <artifactId>swagger-codegen-maven-plugin</artifactId>
                <version>2.3.1</version>
                <executions>
                    <execution>
                        <id>generate-server</id>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>swagger.yaml</inputSpec>
                            <language>spring</language>
                            <configOptions>
                                <sourceFolder>src/gen/java/main</sourceFolder>
                                <delegatePattern>true</delegatePattern>
                                <java8>true</java8>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Start by reproducing the issue with the swagger-codegen Maven plugin, the Spring language, and the YAML schema shown; inspect the generated CustomerApi.java output and the SpringCodegen import-generation path. Done means the generated response still uses Map<String, List> and CustomerApi.java includes the required java.util.Map import so the project builds.