SpringCodegen not importing java.util.List

When using the swagger-codegen-cli and the Spring language, java.util.List is not being imported and compilation fails.

Description

[ERROR] /Users/davidnewcomb/Development/git/x/v1client/src/main/java/com/example/api/MyManagementApi.java:[64,15] cannot find symbol
[ERROR]   symbol:   class List
[ERROR]   location: class com.example.api.MyManagementApi

Swagger-codegen version

3.0.4-SNAPSHOT (branch 3.0.0 HEAD)

Swagger declaration file content or url

Command line used for generation

java -jar swagger-codegen-cli.jar generate  \
	-i liveres-1.0.0-rc2.json \
	-l java \
	-c resttemplate-config.json \
	-o v1client

resttemplate-config.json

{
  "modelPackage" : "com.example.model",
  "apiPackage"   : "com.example.api",
  "invokerPackage" : "com.example.v1.client",
  "groupId" : "com.example.v1",
  "artifactId"  : "v1client",
  "fullJavaUtil" : true,
  "dateLibrary" : "java8",
  "library" :  "resttemplate"
}

Steps to reproduce

After running the generator:

cd v1client
mvn clean install

Compilation fails because file src/main/java/com/example/api/MyManagementApi.java contains lines like:

final List<MediaType> accept = apiClient.selectHeaderAccept(accepts);

but there is no import statement.

Related issues/PRs

Very similar to #7432

Suggest a fix/enhancement

Add import java.util.List to the top.

Reproduce the issue with the provided swagger-codegen-cli command and inspect the generated src/main/java/com/example/api/MyManagementApi.java. Trace the Spring generator output responsible for its imports, then run mvn clean install in the generated v1client project; done means the List type is imported and compilation succeeds.