[Java][google-api-client] Fix bug with empty POST request not sending content-type

[charlescapps]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Java technical committee - @bbdouglas (2017/07) @JFCote (2017/08) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01)

Description of the PR

Fixes #7786

This PR basically fixes a few issues with the google-api-client templates.

The biggest issue is POST requests for endpoints that send empty request bodies are failing with a 415 status code (Unsupported Media Type) because the Content-Type: application/json isn't being sent in this case.

A good example would be -- in our APIs we have lifecycle operations such as POST /foo/{id}/publish or POST /foo/{id}/pause which transition some entity to a different state, but no request body is required since we are simply indicating that a publish is taking place.

This PR also fixes a few random issues with the templates:

• They were failing for query params declared as List<String> or List<Object> of any kind.
  • => fixed this by sending an Object[] to the UriBuilder in the generated code.
• Added an overloaded method to use a java.io.InputStream as the request body.
  • This is incredibly useful for proxy services that forward data, and allows us to use some non-JSON APIs that we have with the swagger client (e.g. text/csv type).

Test Plan

• Generated samples with ./bin/java-petstore-google-api-client.sh
• Did mvn clean test in the samples/client/petstore/java/google-api-client directory and it succeeded
• I have tested this thoroughly before putting up this PR by generating a google-api-client client for a complex API with the new codegen code. I have verified that empty POST requests now work, list query params now work, and input streams work for the request body if the overloaded method is used.

[charlescapps]

Travis failed on some unrelated Python thing, I'm guessing this is a known issue: ERROR: InvocationError: '/home/travis/build/swagger-api/swagger-codegen/samples/client/petstore/python-tornado/.tox/py3/bin/nosetests'

[wing328]

The Travis build error is likely due to a change in the build image. I'll look into it tomorrow.

Thanks for the fix.

[charlescapps]

Thanks @wing328 !