Fix build warning in Java Apache client

Fix build warning in Java Apache client

> Task :javadoc
/Users/williamcheng/Code/openapi-generator3/samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java:210: warning: no @param for objectMapper
  public ApiClient setObjectMapper(ObjectMapper objectMapper) {
                   ^
/Users/williamcheng/Code/openapi-generator3/samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java:222: warning: no @param for httpClient
  public ApiClient setHttpClient(CloseableHttpClient httpClient) {
                   ^
/Users/williamcheng/Code/openapi-generator3/samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java:234: warning: no @param for basePath
  public ApiClient setBasePath(String basePath) {
                   ^
/Users/williamcheng/Code/openapi-generator3/samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java:247: warning: no @param for servers
  public ApiClient setServers(List<ServerConfiguration> servers) {
                   ^
/Users/williamcheng/Code/openapi-generator3/samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java:259: warning: no @param for serverIndex
  public ApiClient setServerIndex(Integer serverIndex) {
                   ^
/Users/williamcheng/Code/openapi-generator3/samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java:271: warning: no @param for serverVariables
  public ApiClient setServerVariables(Map<String, String> serverVariables) {
                   ^
/Users/williamcheng/Code/openapi-generator3/samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java:709: warning: no @param for headers
  protected Map<String, List<String>> transformResponseHeaders(Header[] headers) {
                                      ^
/Users/williamcheng/Code/openapi-generator3/samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java:709: warning: no @return
  protected Map<String, List<String>> transformResponseHeaders(Header[] headers) {
                                      ^
8 warnings

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)