[JavaScript] Allow to use a custom http agent

##### Description
We need to be able to use a custom http agent while requesting our API using swagger generated code.
In our case it is mainly to use keep-alive as we make call to swagger generated code.

##### Swagger-codegen version
2.3.0

##### Suggest a fix/enhancement
Add a `requestAgent` property in ApiClient module if set by user then use it as the request agent